I believe that the 2010 Mac Pro will come in February and will see a straight replacement of the 5500 Gainestown Xeons with 5600 Gulftown Xeons (3500 and 3600 respectively).
Systems will be pretty much as they are now just with 6 and 12 cores instead of 4 and 8 cores. This will provide a clear segment differentiator between the consumer and the Pro range. The other technical upgrade will be 10G Ethernet. I think Apple will push the prices up another notch and shift even more Mac Pro customers to the iMac and Mac mini.
The iMac could get on die graphics and an on die chipset on top of the 4 cores in 2010 or 2011. It might even get an on board SSD on the logic board and eSATA. Matte displays could be optional as they are now in MBPs. Why not? It would make a serious dent into the Mac Pro sales.
Sandy Bridge in 2011 will be a decisive year and then Haswell again in 2013. If there are still enough customers left they may do one or more tick tock cycles. If not they may abandon the Pro line which IMHO will happen in the long run anyway. Windows has a much bigger share of the workstation market and the vendors offer systems with higher performance and cost efficiency for most professional uses. That will not change and necessarily will lead to Apple giving up. Whether that will happen in 2011, 2013 or 2015 will depend of the market dynamics.