In researching a replacement hard drive for my 15" CD MBP, I found that as long as it's a 2.5" SATA drive it should work.
Looks like you can go up to around 160GB on a 2.5" SATA drive these days, however, there are caveats -
1.) The larger the drive, the lower the speed. The largest drives are running around 4200 rpm
2.) 100GB drives and smaller can be had up to 7200 rpm.
3.) The stock 100GB drives Apple supplied are 5400 rpm.
So it's a tradeoff. If you want speed, you're probably going to be stuck with 100GB or smaller. If you want space, you're going to go up to 160GB, but sacrifice speed in the process.
Depending on what you're doing with your MBP, this might not be an issue. In my case, I do a lot of video field editing with FCP, so I need speed. In a worst case scenario, I use an external FW drive for a scratch drive, but in most cases I can get by with a 100GB until I get back to home base and my editing suite.
If you don't need the speed, you can gain the storage space. It is noticeable based on the machines I've seen that have different speed drives, but not significant for most applications.
I shopped around and was able to find a brand new Hitachi 100GB 7200 2.5" SATA drive on eBay for $100. From what I saw this is around the bottom end of the price range these days.
MD