Intel's latest roadmap reportedly includes higher front side bus speeds for Penryn-based Xeons, according to DailyTech.
The site lists 3 processors slated to include a 1600 MHz front side bus, up from today's 1333 MHz front side bus, 2 of which are quad-core. The E5472 and E5462 are clocked at 3.0 GHz and 2.8 GHz respectively, and will feature Penryn's 12 MB L2 cache previously described.
Since transitioning to Intel, Apple has elected to put the highest-end Xeon 51xx ("Woodcrest") and 53xx ("Clovertown") series processors in its Mac Pro line. Quad-core Penryn-based Xeons will be using the 54xx numbering scheme, and are expected to be used by Apple.
Apple has also twice used unreleased processors in its products, so a Penryn based Xeon based on the new bus speed faster than 3.0 GHz should not be discounted at this time.