Well, the companies strive to be predictable for their customers, so according to their roadmaps..
Now until Q3 09 - Nehalem and Shanghai
Q4 09 to late 2010 - 32nm Westmere quad or hexa-cores, Shanghai (octocores for servers from both manufacturers), and then there's Larrabee, the wildcard.
2010 is the year of Sandy Bridge, a revolutionary rather than evolutionary core. It'll be more similar to the Cell and GPUs than current CPUs are. GDDR/throughput memory, ring bus, AVX, incredible floating point capability for a CPU.. AMD has a similar concept in Bulldozer, but they haven't said too much about it and it might be delayed to 2011.
Essentially, a Nehalem system is going to be very competitive until 2010, and even Harpertown and Yorkfield can cut it for quite a while on power alone, if not bandwidth.. But in 2010, they go out the window. Too slow.
Then there's the chance of AMD making a 'right-hand turn' similar to Intel's 'right-hand turn' going from NetBurst to Core, and changing the deck entirely.