The Mac Pro is currently "unavailable" both at the Apple Store and many physical retail locations around the U.S. While the news is by no means a confirmation that an update is near, the stars are certainly aligning.
As previously reported, the Mac Pro line was rumored to be on the receiving end of a refresh in June. Obviously that did not happen. Then further update evidence popped up in in Mac OS X, where there was evidence of high-end graphics card support (in this case the usual ATI and NVIDIA suspects). Another report outlined support for USB 3.0.
The current Mac Pro refresh buzz says an end of summer release is likely. Coupled with today's MIA Apple Store listings, that seems all the more likely. Stay tuned? [MacRumors]