This is what you need to do. I've tested this on a 17" and verified it working, it was a personal project of mine when multiPod Touch came out. you need a top case from the 2008 "Penryn" MacBook Pro, preferably with a keyboard on it already. if no keyboard, you'll have to swap yours onto it. on the bottom of the top case there's a cable that connects to the logic board, this is how the computer communicates with the top case, trackpad, keyboard, and temp sensors. you're going to have to swap this cable and temp sensor with the one from your current top case. once you have the new top case with the old cable and old temp sensor, you can install it on your pre-multiPod Touch MacBook Pro. There is zero configuration once you boot up; since the trackpad is a USB device, it works immediately. in response to what Markus said about 4-finger swiping, the Penryns got that feature once Snow Leopard came out, so if you're on 10.6 or newer you're good to go.
feel free to PM me about this, I had a lot of fun figuring this out since apparently I was the first to do it 100% successfully (I couldn't find any supporting info on the interwebs about it).