Apparently, MacBooks Pro unibody 15-inch officially support up to 8GB of RAM, unlike the 4GB announced by Apple itself:
The company started selling 8GB upgrade kits on its online store, also for the smaller model, previously only available for the 17-inch MBP and Macs Pro (in addition to the new iMacs). The most basic model of all, 2.4GHz, the only one not contemplated.
The pair of combs (2x4GB 1,066MHz DDR3 PC3-8500 SO-DIMM) costs $ 1,200 at the Apple Store Online, same price as other machines (almost a MacBook unibody basic!). Unless you buy directly from there with the upgrade, it is still worth investing in it from the outside, where it is usually much cheaper.