Apple's likely looking to unify their gui rendering path to the greatest degree possible, and moving things OpenGL is the easiest way to do this and have things still be nice and fast. They might might lose some on a $10 or $15 increase to the BOM on a laptop, but the potential savings in engineer hours might be tremendous.
There's also the chance that a desire to use OpenCL more widely for internal applications means that having a gpu that can run CUDA or the equivalent is becoming a defacto requirement for all macs.
Laptops are increasingly people's primary computer. If you're going to buy a laptop anyway, the question becomes do "I even need a desktop?" If the laptop acceptably plays WoW or whatever other popular games, that gives it a significant advantage.
Between all major desktop operating systems offloading more and more desktop graphics via OpenGL, and the increasing availability and those same desktop operating systems adding GPGPU APIs, the GPU really does directly affect desktop performance.
http://www.brightsideofnews.com/news/2009/12/5/apple-ditches...
If this rumor is true, it is quite an extreme move. Is 3D graphics that important nowadays? I thought most laptops were bought for work, not games.