Seems a few months too soon for the Mac Pro. Apple doesn’t just pull new Macs out of thin air when they feel like it: they usually follow Intel’s roadmap closely, especially for the Xeon-based Mac Pro.
What’s happening in the Xeon line in July or August to motivate a Mac Pro update? As far as I can tell, nothing. The single-socket Xeon was recently updated to Sandy Bridge with the very good E3 line, but the dual-socket E5 line isn’t due out until the fourth quarter.
So, in descending order of likelihood: either this rumor’s timeline is wrong, Apple’s getting very early access to the Xeon E5, or the next Mac Pro isn’t offering any dual-socket models at launch.
(I can’t speculate on the Mac Mini on this basis, since it rarely uses a brand new CPU and usually is pulled out of thin air when Apple feels like it.)