There's nothing like pushing a new major update in order to find bugs 
in the code when its exposed to a couple of hundred working 
applications. Thankfully the fixes were almost as swift as the reports. 
In any case, you'll _definitely_ want to upgrade to 0.12.1 right away. 
There's a good handful of fixes for both Action Pack and Active Record 
(mostly concerning the new eager loading).

Here's the dirt, so you don't have to go look it up. First for Action 
Pack:

* Added xml_http_request/xhr method for simulating XMLHttpRequest in 
functional tests #1151 [Sam Stephenson]. Example: xhr :post, :index
* Fixed that Ajax.Base.options.asynchronous wasn't being respected in 
Ajax.Request (thanks Jon Casey)
* Fixed that :get, :post, and the others should take a flash array as 
the third argument just like process #1144 [rails / cogentdude.com]
* Fixed a problem with Flash.now
* Fixed stringification on all assigned hashes. The sacrifice is that 
assigns[:person] won't work in testing. Instead assigns["person"] or 
assigns(:person) must be used. In other words, the keys of assigns stay 
strings but we've added a method-based accessor to appease the need for 
symbols.
* Fixed that rendering a template would require a connection to the 
database #1146

Then for Active Record:

* Fixed frivilous database queries being triggered with eager loading 
on empty associations and other things
* Fixed order of loading in eager associations
* Fixed stray comma when using eager loading and ordering together from 
has_many associations #1143

Updating, as always, couldn't be easier than gem install rails 
--include-dependencies
--
David Heinemeier Hansson,
http://www.basecamphq.com/   -- Web-based Project Management
http://www.rubyonrails.org/  -- Web-application framework for Ruby
http://www.loudthinking.com/ -- Broadcasting Brain