WordPress 3.2 requires MySQL V5
WordPress released V3.2 and with it, the requirement to run mySQL V5.
To date, most open sources solutions have been backwards compatible with regard to MySQL. For instance osCommerce will run on both V4 and V5.
However with the 3.2 release, WordPress makes MySQL V5 mandatory.
Many hosting companies have already upgrade to phpV5 and are in the process of upgrading to MySQL V5. Unlike php, it is problematic to offer both versions on the same server. So transition is not simple.
ozEworks will be upgrading its servers to MySQL V5 in last quarter 2011.