e-con systems (India) has a camera module that plugs straight into the J5 connector on the Gumstix (Overo)
Check out the specs on the page, it is a full auto focus module and seems to have some pretty nice specs.
I bought one of these, and although I'm still very much a novice I can confirm that it takes pretty good pictures (even in low light), and I just got video recording working too. Note that you need to buy the flexible cable from www.gumstix.com, as e-con don't supply this.
If you look into using this and have any success, post it on here, as I'm still learning about it myself!