Since Intel lists the fastest quad Xeon as the X5355 at 2.66 GHz, it looks pretty obvious that Apple is overclocking the 2.66 chips.
Intel has a utility to compare the actual clock speed to the rated speed (Windows or bootable) http://support.intel.com/support/processors/tools/frequencyid/ - so soon we'll know if Intel produced a special run for Apple, or if the Cupertino boys and girls have been "mod'ing"
http://www.intel.com/support/processors/tools/frequencyid/sb/cs-007627.htm
This looks to me like Apple have been given access to the 3Ghz clovertowns early and have decided to add them to the range.