Are you looking for something low-cost or more of a high-end managed service?
MediaTemple has RoR grid containers and other hosts support Rails now, but I don't know of a turnkey solution ... although Heroku looks interesting.
Seriously, if you get a slicehost slice w/ Ubuntu Hardy, Apache and mod_rails, you can get your app up in < 3 hours.
Plus, there's something to be said for getting on the server and learning how to do some things yourself.