I'm waiting for quad core MBP's and 8GB of ram to be cheaper. My SR 2.4ghz 17" is working great still. Current MBP's aren't much of an upgrade IMO.
Quad Core chips in the MBP and dual in the MB would differentiate the pro's from the non-pro's very well. It makes the most sense. It won't happen until the Mac Pro is 8 core standard though. Quad Core MBP would probably be close to as fast, in some things, as the current 4 core Mac Pro.
Hopefully by 2010 the Quad Core MBP's come out. That's when my Applecare expires!