I needed to get my MacBook Pro running again after the hard drive failed, but the only drive I had available was a desktop 3.5 inch SATA unit.
So I opened up the MacBook Pro to see what could be done. Unfortunately, the onboard SATA connector won't accept a standard SATA cable. However, all you need to do is chop off some of the excess plastic on the SATA cable, and it works.
Note that you do need an external power supply for the hard drive, as the internal source doesn't supply the full 12v required.
Pro: Backup system is viable
Con: No longer a laptop
[robg adds: I haven't tested this one, and hope I never need to do so!]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20090425125846422