Posted to the REE Google Group as well: https://groups.google.com/forum/#!topic/emm-ruby/EBkMfFpk40A
A week back I innocently updated Sinatra on an app which in turn updated Rack. Since then, I've had crazy hung passenger processes that just gobble up CPU like it is going out of style.
After spending a few days trying everything I knew to to fix it, today I got help from a friend (Eric Lindvall) and dug in with strace, rbtrace, gdb, and gdb.rb and found the issue. Rack 1.3.0 tests a regex against a URL and it causes things to hang.