I just ran into a client who’s WordPress installation broke due to the 3.3 upgrade.
The fix is easy, you need to install WordPress 3.3.1 (when it comes out). For those of you who need this fixed immediately, you can install the Hotfix 0.8 plugin. It will disable the JSON support that is breaking in 3.3.
UPDATE – 2012-01-03
WordPress has released version 3.3.1 which is supposed to fix this issue.