Please, please Search the forums. We answered this 3 days ago... and every week since the C2D MBP was released....
You can put 2 x 2Gb in the MacBook Pro if you like. THe chipset will only recognize 3 Gb of it. Same deal with Windows Core2Duo laptops - put in 4 Gb and you'll get 3.3 Gb or less.
It's the chipset. It will not be fixed with an OS upgrade nor a firmware upgrade. It is what it is.
I've been told from somewhere that all a MBP can use is 3.2 GB, keep in mind, that is a lot of RAM, unless you're some sort of professional cinematographer or something, but then you'd have a Mac Pro. But is 4 GB really necessary? Sure it'd be nice (don't forget expensive too), and it would let the computer last a little longer in the stomach flu of technological advancement that vomits out a new and improved product every 6 months or so. Sorry, got off on a tangent. 3.2 GB is about as much as you'll get out of the current MBPs, and .2 GB isn't worth the $$$ (apple sells 'em fer ~$500) for that 2 GB chip you won't really use. Which is why they only sell 3 GB. Heck, I'd love 4 GB of RAM, but my wallet wouldn't .