I have worked as a C++ developer for a couple of years and I don't have much experience with Cocoa, but from what I've been reading yesterday regarding Grand Central Dispatch, it seems very easy (or a lot of easier) to incorporate multicore support into your applications. Its a very well designed and smart system.If you are running an OS with SMP support, it will schedule threads onto multiple processors and you will see a benefit from multiple cores whenever you are multitasking. Even though OS X was fairly late to the SMP game it's been capable of doing this reasonably well for a good 2-3 years now.
At first I had some reservation at a possible quad core iMac (I had a Quad Core PC which was very underutilized before my imac) , but now it seems to make perfectly sense to equip macs with more cores.