Thought I'd post this for people like myself who don't have Xbox Live and therefore couldn't get the backwards compatibility downloads for their harddrives.
Go and buy the new issue of OXM (Saint's Row is the feature on the disc cover). On the demo disc is a download that will instantly update your harddrive so that you can play all of the currently compatible Xbox games on your 360.
You can go to Xbox.com for a list of all of the currently playable games.