Whoa, I was just moaning this morning about the lack of PBBGs using Rails :)
Pioneers of Aethora (http://aethora.com [aethora.com]) is written in Rails. Just to chime in, once you get the hang of Ruby, most people really like Rails. There are some performance considerations to be aware of, but if you stay on top of it, it can be managed.
Thanks for the article, walski.