Heroku! Basically the Ruby equivalent of GAE—you get a Postgres database instead of Bigtable, though (whether that's a good or bad thing is up to you.)
This may warrant a separate question, but I've seen various posts here and there about running Ruby/Rails on GAE using JRuby. Does anyone have any experience as to just how realistic that is as a deployment option? I can imagine it being full of headaches, but I would love to hear from someone who has either tried it or is actually running it on a production app.
That config is basically only for development/prototypes, though (it has a 5MB database cap); their default tiny-ish plan is $15/month, which is quite decent for anything that runs Ruby, let alone on a grid.
Turning on all the knobs and levers on that page gave me "$5089/month", which is a slight bit terrorizing—but I can't imagine the kind of business that would grow to need that scale without becoming a multinational with its own IT.