Last week at the OUG Harmony conference thanks to Dimitri Kravtchuk I learned that setting query_cache_size=0 does not disable and remove locking from the Query Cache. You actually need to also set query_cache_type=0. This appears to been a bug, seen in the presently still open MySQL bugs database entry #38511.
My recommendation to customers now is to set both variables on all existing MySQL versions if you are not using the MySQL Query Cache.
Thanks to the Performance Schema in MySQL 5.5 for uncovering this. More information in Dimitri’s detailed post at MySQL Performance: Using Performance Schema
Details of all MySQL presentations at OUG Harmony 2011 in Helsinki, Finland.