If you have Plesk 8.1 or later, you have support available for Ruby on Rails. Unfortunately, clicking the FastCGI checkbox in Plesk won't get you all of the support you need (and expect). The folks over at Parallels created a relatively simple process to get Ruby on Rails working properly on your site:
Go to your domain that you want to adjust, and click Setup. Make sure the CGI and FastCGI options are enabled. Pick a name for your application and make the directory for your application in the httpdocs directory. Upload your files to that directory.
Once you've done that, create an .htaccess file in the httpdocs directory with the following text inside:
RewriteEngine On
RewriteRule ^$ /public/index.html [L]
RewriteCond % !^/railsapp/public
RewriteRule ^(.*)$ /public/$1 [L]
RewriteCond % !-f
RewriteRule ^(.*)$ public/dispatch.fcgi/$1 [QSA,L]
Remove the .htaccess file within the public directory of your application and add a file called dispatch.fcgi to that directory which contains:
#!/usr/bin/ruby
You should be able to access your application at http://domain.com/railsapp/.
I've found the ruby support to be very lacking (or I am missing something). How are you handling installing add-ons, gems, etc? I've often had to re-map ruby to a new directory and install later versions to get the support the clients want.