What's funny is that a few months ago, I was able to bring my brother's '06 MP into the local Apple store to have the Airport Extreme card installed for $44 (installation included). So, I bought a '08 MP Octo on clearance a couple weeks back and now none of the stores seem to stock the part anymore. They wouldn't order the part, either.
So I picked up the card for $49 at J&R and installed it myself. Connecting the tiny wires to the terminals on the end of the card is a PITA and it takes lots of patience. But after about an hour, I was back up and running with Wi-Fi. I'm pretty handy tech-wise, but if you're not comfortable with working in small spaces with delicate wires, I would recommend taking it into an Apple service center to get it installed for a charge. The card itself isn't hard to insert into the socket, but connecting the antenna wires to the card is.