You'll have to wait until the Arrandale processors are released which is due around March 2010, although going on Apples previous track record with Intel, Apple should get them around early February, before I would have said in time for Macworld, but obviously since Apple doesn't attend that anymore thats unlikely. I'd still get a Core 2 Duo though until about October 2010 simply to maximise value because otherwise your getting the worst of a new thing if that makes sense.
Also to people who are saying the iMac's will get Quads before the Macbook Pro's, no they wont, the iMac is at the bottom of the food chain along with the 13" Macbook before it became the 13" Macbook Pro which it really isn't because it isn't a Pro machine.