I really think it's due to Apple's purchasing tactics. They order tons of a component (which helps them in the iPod nano department, they've ordered 40% of Samsung's output) and like Luca said, they won't update until those parts bins are empty.
PC laptop manufacturers can order in the same quantities as Apple but they'll run out of those parts sooner due to volume. This makes it easier for PC laptop makers to update their components. Also, there is much competition on the PC side, while on the OS X side there is only one manufacturer of OS X capable computers so there is much less pressure to update their offerings.
I've been digging around PC laptop sites and unless you buy a top of the line laptop, a 7200 RPM drive is an upcharge item. Most laptops still come stock with 5400 RPM drives.
As far as GPUs go, Apple seems to always get the short end of the GPU stick. There just isn't the huge market for go fast parts on the Apple side compared to the PC side. Plus you still pay a huge upcharge for a current GPU. Some GPU upcharges are close to $300 if you want something close to the latest and greatest.
The GPU VRAM thing is all related to Apple's product crippling strategy. You can't have the mini with 128MB VRAM when the iBook still has 32MB or whatever. They'd have to increase VRAM across the board which leaves them with a bunch of full parts bins they'd have to scrap.
Remember, these obsolete GPUs may be off the market for new buyers, but nVidia and ATI still have to honor their purchase contracts with Apple. So even if you can't buy a Radeon 9200 32MB in the store (who would want one anyway?) they still have to fill Apple's order.
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