Yep, it absolutely works, at least on my dev box.
I'll have to go back and check but I thought the BillMonk guys mentioned something about building that part outside of Rails for performance reasons.
You can replace any part of Rails for performance reasons. Rails is not built to be the highest performing solution on the market. The question is if you want to spend your time doing optimization or features. Personally I prefer features until a need for optimization shows up.
I thought I would bring it up now while I'm still early enough into the project.
Well, if you're going to be handling big amounts of mail it might be worth going outside of Rails. Try doing some benchmarks and see what you get. Or it might be worth just getting more hardware. In the end you should do what you think is right