The rumours are in fact pretty clear about the next Pro update.
Since suitable Sandy Bridge processors are scheduled due Q3/4 2011, it is very likely that the Pro will be updated in Q1 2012.
Yeah +10. There's really nothing to upgrade the Mac Pro with, no parts. Unlike Apple, Intel is pretty clear about their near-term future roadmap and expected dates of delivery on new tech. They may speed bump the base configs from dual hex @ 2.66 & 2.93 to dual hex @ 2.93 & 3.33 (sorry dunno offhand what the other MPs are in the current lineup). But nothing too exciting.
Whatever happens there isn't going to be any great fanfare, or an event like the iCrap gets, or even a 5 second mention at WWDC. They'll update the Mac Pro page and the people who need 'em, will notice and everybody else will continue ignoring it. The release will not even get a 2 line text mention on Apple's front page.
Going from a G5 to any Mac Pro is going to be like night and day, waiting until Q1 2012 (which may be Q2 or Q3 even, the 2010 MP's were what, 6 months later then expected?) is kind of waiting for the sake of waiting. Apple's priority is, in order of importance: all the iCrap, their laptops, the iMac, the Mac Mini, a revolutionary new and improved LED blinky light on their magical battery charger, and then whenever they get around to it, the Mac Pro.
It'd be nice to see LightPeak in the hypothetical new Mac Pros (Actual Release Date: Q4 2012!), but somehow I strongly doubt that any new tech is going to be introduced in the MP, it'll show up in the MBPs and iMacs before MP ever gets it. OTOH someone will release a LightPeak card 3 weeks after the tech is available.
The last thing to keep in mind is, so far as I know, the BuyersGuide buying guidelines and historical length of time between release dates, is counting backwards to a mythical time that predates all the iCrap, and even the iPod, when Steve Jobs used to go on stage and demo the new Mac Pros and proudly proclaim that they'd pass 3.0GHz Real Soon Now! Uhm, those days are long-gone, and my stock thanks Steve for that, while my heart curls up into a fetal position and cries tears of sadness and empty neglect. <blowing nose>. Anyhoo, what I'm saying is, the Buyers Guide as it relates to Mac Pro release dates, is a little off ... like by about a year almost at this point, 9 months certainly.