ThinkSecret believes that Apple will start to receive Blu-ray drives from Sony in February. While the site has previously reported that Apple may also back rival format HD-DVD, this information perhaps pegs Blu-ray equipped Mac Pros early next year.
The information is tempered by the fact that ThinkSecret has not been extremely accurate in recent history. In addition, AppleInsider currently believes that the Mac Pro is due to receive an update before the end of the year, which would make an early-2007 update to the Mac Pro 3 updates in half a year, which is an unusually fast sustained-pace for pro-system upgrades.
While the possibility certainly exists that Apple could introduce Blu-ray functionality in a system other than the Mac Pro first, and indeed ThinkSecret leaves that door open by not specifying which Mac would start shipping with the drives first, most educated speculation points to Apple's pro systems being the first to benefit from the upgrade.