The Mac Pro is currently "unavailable" both at the Apple Store and many physical retail locations around the US. 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 rumoured 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 a mid-year release is likely. Coupled with today's MIA Apple Store listings, that seems all the more likely. Stay tuned? [MacRumors]