So, after completing a move to Edge Rails today, let me give you one piece of advice:

Explicitly declare your session options, and don't forget session_key. Without it, the new cookie performance enhancer blows up unceremoniously, and unless you want to spend a half hour searching dev.rubyonrails.org, you won't know why.

So, for example, in ApplicationController, you'd need:

  session :secret => 'abracadabra', :session_key => 'myapp_session'

And, voila! Problem solved.