What is Mod_Rails (Passenger)?
Publicly released on 11 April 2008, mod_rails (a.k.a Passenger, built by the Phusion team Hongli Lai and Ninh Bui) allows Ruby on Rails deployment to be as simple as uploading your files.... and you're done.
Why would I want to use mod_rails for deploying my Rails apps?
Early tests of mod_rails have shown it to be faster and as equally robust as Mongrel. Therefore, mod_rails is a simpler method of deploying even your largest Rails apps while theoretically achieving better performance (this has yet to be globally tested/accepted). Best of all, you don't need to worry about separate application servers and proxy balancers to load/restart/setup/monitor/etc...
How does mod_rails (Passenger) compare to Mongrel in a shared hosting environment?
The jury is still out, but Passenger 2.0RC1 along with Phusion's "Ruby Enterprise Edition" (Both released 9 June 2008) is significantly better in terms of VM size than 1.x w/ normal Ruby. At this point, however, we still believe that Mongrel and Mongrel clusters will prove to be more stable in a shared hosting environment. When mod_rails processes die off there is still a short delay as they fire up again (similar to what happens with FastCGI instances)
How can I deploy my Rails applications with mod_rails (Passenger) on HostingRails.com?
Upload your app, symlink RAILS_ROOT/public to your account's ~/public_html and contact support to configure your apache vhost to use mod_rails. That's it. Beginner and Basic shared hosting plans can only run a single mod_rails app, and Professional plans and up can run as many as will fit within the VM size purchased for the account. More detailed tutorials for deployment and Capistrano integration are on the way! (you can also read the mod_rails docs).