This is assuming you know the rest of the rails application stuff: putting the files somewhere 
and configuring database.yml (and the requisite setting up mysql and mysql users).

1) Put in a mongrel_cluster.yml file that will pull your private IP automatically, and write
   hostname-specific PIDs.

# cd /home/YOUR/PATH/TO/RAILS/APP
# svn export http://svn.joyent.com/public/accelerators/configs/mongrel/mongrel_cluster.yml config/mongrel_cluster.yml

2) Fire up the application

# cd /home/YOUR/PATH/TO/RAILS/APP
# mongrel_rails cluster::start

3) Check that you're getting something

# curl `myprivateip`:8000
# curl -I `myprivateip`:8000

The second one should return a "200", like

	HTTP/1.1 200 OK
	Connection: close
	Date: Tue, 06 Mar 2007 07:09:13 GMT
	Status: 200 OK
	Server: Mongrel 1.0.1
	Content-Type: text/html
	Content-Length: 0

If there's nothing happening check your production.log

4) Configure an apache virtual host to proxy balance to these mongrels

# cd /opt/csw/apache2/etc/virtualhost
# nano myrailsapp-domain.conf

With the contents of

<VirtualHost YOUR-PUBLIC-IP:80>
ServerName YOURDOMAIN.com                                                                                                                                                                       ServerAlias *.YOURDOMAIN.com 
DocumentRoot /home/YOUR/PATH/TO/RAILS/APP/public                                                                                                                                                                            
<Directory "/home/YOUR/PATH/TO/RAILS/APP/public/">                                                                                                                                                                               
Options FollowSymLinks                                                                                                                                                                                                  
AllowOverride None                                                                                                                                                                                                      
Order allow,deny                                                                                                                                                                                                        
Allow from all                                                                                                                                                                                                        
</Directory>           
<Proxy balancer://app1-mongrels>                                                                                                                                                                                   
BalancerMember http://YOUR-PRIVATE-IP:8000                                                                                                                                                                                    
BalancerMember http://YOUR-PRIVATE-IP:8001                                                                                                                                                                                    
BalancerMember http://YOUR-PRIVATE-IP:8002                                                                                                                                                                                    
BalancerMember http://YOUR-PRIVATE-IP:8003                                                                                                                                                                                    
</Proxy>                                                                        
ProxyPass /images !                                                                                                                                                                                                     
ProxyPass /stylesheets !                                                                                                                                                                                                
ProxyPass / balancer://app1-mongrels/                                                                                                                                                                              
ProxyPassReverse / balancer://app1-mongrels/                                                                                                                                                                       
ProxyPreserveHost On  
</VirtualHost>

# apachectl restart

You need to have a unique name for "balancer://app1-mongrels" and the ports you use are the same that are in your mongrel_cluster.yml

5) Now make sure that you're app is under SMF and will restart on reboot

Replace YOURAPP, the /HOME/YOUR/PATH/TO/RAILS/APP, and the USER and GROUP


<?xml version='1.0'?>
<!DOCTYPE service_bundle SYSTEM '/usr/share/lib/xml/dtd/service_bundle.dtd.1'>
<service_bundle type='manifest' name='mongrel/YOURAPP-production'>
  <service name='network/mongrel/YOURAPP-production' type='service' version='0'>
    <create_default_instance enabled='true'/>
    <single_instance/>    
    <dependency name='fs' grouping='require_all' restart_on='none' type='service'>
      <service_fmri value='svc:/system/filesystem/local'/>
    </dependency>
    <dependency name='net' grouping='require_all' restart_on='none' type='service'>
      <service_fmri value='svc:/network/loopback'/>
      <!-- need nfs/client because the mongrel stuff is on /home which is nfs mounted -->
      <service_fmri value='svc:/network/nfs/client'/>
    </dependency>
    <dependent name='mongrel_multi-user' restart_on='none' grouping='optional_all'>
      <service_fmri value='svc:/milestone/multi-user'/>
    </dependent>    
    <exec_method name='start' type='method' exec='/opt/csw/bin/mongrel_rails cluster::start' timeout_seconds='60'>
      <method_context working_directory='/HOME/YOUR/PATH/TO/RAILS/APP'>
        <method_credential user='USER' group='GROUP' />
        <method_environment>
          <envvar name="PATH" value="/usr/bin:/bin:/opt/csw/bin" />
        </method_environment>
      </method_context>
    </exec_method>
    <exec_method name='stop' type='method' exec=':kill' timeout_seconds='60'>
      <method_context/>
    </exec_method>
  </service>
</service_bundle>

Put this xml file at

# nano /HOME/YOUR/PATH/TO/RAILS/APP/mongrel-app-smf.xml
# cd /HOME/YOUR/PATH/TO/RAILS/APP/
# mongrel_rails cluster::stop
# rm log/*.pid
# svccfg import /HOME/YOUR/PATH/TO/RAILS/APP/mongrel-app-smf.xml
# svcadm enable mongrel/YOURAPP-production
# svcs -v | grep mongrel