This mod is quite simple on MacBooks, as the system will recognize the new (relatively cheap) top case and maps keys accordingly. On a MacBook Pro, however, unless you change the whole top case (preferable, but much more expensive, and possibly impossible with the oldest MacBook Pros, as they have Bluetooth inside the bottom case), the system gets the needed product ID from the top case. So a software mod is needed.
After some digging in the system, I found the file in question: /System » Library » Extensions » AppleUSBTopCase.kext » Contents » PlugIns » AppleUSBTCKeyEventDriver.kext » Contents » Info.plist. Don't forget to back it up first, and then modify it as super user.
Go to System Profiler and find the product ID; it's located Hardware » USB » Apple Internal Keyboard / Trackpad » Product ID. For example, on my European late 2006 MacBook Pro Core 2 Duo, it's 0x021b. Convert that value to decimal, using Calculator (View » Programmer first, then Hex). That's the Product ID of your top case (mine is 539).
Next, find a section in the file which belongs to the MacBook Pro (Early 2008). Simply search for 560 (US), 561 (Europe), or 562 (Japan), and copy the string belonging to a key FnFunctionUsageMap within the same dict (it comes about 15 lines before the ProductID). The string looks like this: 0x0007003a,0x00ff0005,0x0007003b etc. Note that this string even makes sense: every second value is a function, e.g 0x000C00CD means play/pause, and it is only in 560-2 sections, while 0x000C00E2 means mute and it's there more times, 20th (F10) in the same section but already 6th (F3) in section 539.
Then find a section which belongs to your MacBookPro (again, in my case, it was some lines before 539) and replace the FnFunctionUsageMap string with the string from step two. Finally, delete /System » Library » Extensions » Extensions.mkext, and restart your computer.
The keys should work now, even their alterations -- Command-Exposé or Control-Exposé, for example, or Alt and most keys to bring up their System Preferences panels. Please note that you're modifying system files and these can be also rewritten by future software updates. Happy modding!
[robg adds: Proceed at your own risk with this one, clearly! It's a nifty modification, but definitely not for casual users.]

