The OP posts a reasonable question that can be answered without speculating about an exact new MP introduction date.
In December it is appropriate to wait to see what happens at Mac Expo SF in January prior to making major Apple purchases. A plethora of new hardware/software of all types from scores of vendors is introduced at Mac Expo. The Mac landscape always goes through a major evolution whether or not any one specific product is updated.
E.g. note that even if new Mac Pros somehow were not introduced, new graphics cards and/or hard drive solutions will probably become available that would affect the configuration-optimizing of a tower purchase.
I strongly recommend Mac Expo SF to all. Hundreds of Mac vendors and the opportunity to individually talk to folks high up the food chain makes a couple of days there a good annual grounding for understanding the state of the Mac world.
Important other non-Mac vendors also present (e.g. wanna handle a New Nikon D3 or D300; or bitch out one of Microsoft's Office engineers; or personally test Epson's latest printers, scanners?). Often engineers and product managers will, one-on-one, advise the future direction that products are going, the kind of info never published.
Watching the very latest product demos by great presenters on optimized hardware can provide really good info as to the relative competence of different apps. One app may shine while a competitive app falters, allowing prescient insight into which app one should commit to. E.g. folks considering modernizing DSLR workflows should carefully watch the Aperture and Lightroom presentations.
-Allen Wicks