Also, I know that Rails 2.0 there's a new request profiler. See
Ruby on Rails — Rails 2.0: It's done! and scroll
down to the part about the new request profiler.
Personally I’ve had a lot more success using DTrace to profile Rails apps. You can even attach to an existing process, so you can trace Passenger servers. You need to be running Mac OS X (Solaris may work too, but I don’t know) but most Rails devs do so…