An issue I encountered recently doing a WordPress upgrade from 3.1 to 3.2, is that afterwards everything appeared to work with the exception of the editor for Posts and Pages. After working through the checklist for the manual install and checking each plugin individually, I took a look at the PHP error log and noticed that PHP didn’t have the JSON extension bundled.
If you get an error mentioning that json_encode() is missing, it will be because PHP wasn’t compiled with it and WordPress now uses it. If you use SuSE (which this particular server is) you need to install the php5_json package using a command similar to:
zypper install php5_json
Otherwise you will need to see if your distro has a JSON-specific package for PHP, or you may have to compile it from source.
NOTE: PHP usually has the JSON extension compiled into it by default since 5.2, so this is an unusual case