MacinDoc is correct. If you want to avoid voiding the warranty, an ExpressCard is your only option. (Although you can do it without buying the whole router.)
If you're willing to open your notebook, you can try to find Apple part number 661-4060 or 603-9452-A or MA688, (I've seen all three,) which is for the Mac Pro AirPort card. It uses the same physical connector inside the computer (mini PCI Express,) and has the same antenna connectors as the Core Duo MBP, and Mac OS X recognizes it as a valid Apple 802.11n card just fine.
Installing it may or may not void the warranty. Should you have issues with the card installed, I would definitely put the original card back in before taking the computer to Apple.
Unfortunately, even though the list price for the Mac Pro AirPort card is only about $49, stores that are willing to sell it to you 'bare' charge a noticeable markup. (I've seen it for over $100.)