I have tried various combinations and find that by simply generating a skeleton Rails 3.0 application, and then running
siege http://localhost:3000
My ruby process would then leak a few MB every 5 minutes.
The reason I tried this is because I just converted a full blown Rails 2.3 and Ruby Enterprise 1.8.6 application over to Ruby 1.9.2 Rails 3.0 Beta 4 and find that my app is leaking memory.
This happens for both development and production environment, however happens a lot faster in development.
I do not think this is a specific Rails 3.0 problem because I am also experiencing the same with Rails 2.3.5 and Ruby 1.9.2
So it is probably how Ruby 1.9.2 is interacting with Rails.
I never had such problem with Ruby Enterprise 1.8.6.