A customer opened an issue recently to ask why the query cache wasn't working after he upgraded to MySQL 5.5.25. The reason really ended up surprising me.
As of MySQL 5.5.23, the Query Cache is disabled for partitioned tables!
This is a "fix" for bug #53775.
At first I thought perhaps the fix for the bug had resulted in the query cache being inadvertently disabled for partitioned tables, but the comments that go along with the commit make it pretty clear that disabling the query cache was the intended "fix". You can review the commit message and the code changed at revision 2661.803.1 in the MySQL Server 5.5 repository.