Hi,
This bit me but I haven't seen it mentioned on here yet. I have Rails vendored as a submodule so to upgrade from the release candidate to 2.3 I simply pulled and checked out the v2.3.2 tag (73fc42cc0b5e94541480032c2941a50edd4080c2).
Unfortunately a rather large bug means the nested parameters will not be hashed when POSTing, e.g. user[name] will be "user[name]" => "bob" instead of :user => {:name => "bob"}. This is because the gem install of rack takes precedence instead of the newer bundled one, it's fixed in the immediate commit after (39ff550fa88da9a22d8c21ca872f5e4d0d83f8d4). Pulling from the HEAD of master is pretty safe at the moment anyway.
Thanks to Matt Jankowski for original report: http://rails.lighthouseapp.com/projects/8994/tickets/2259-params-hash-issues
Hope this helps someone avoid trouble!
Andrew