Disclaimer: I don't have the app, so I don't know for sure...

I think the easiest way to solve this problem is to read the letters available, figure out how many letter the solution is, then compare these to the database of DrawSomething words. Many times you'll get a handful of possible solutions (especially for shorter words), but often enough, there will only be one possibility.

I'd be impressed if it used computer vision. I'd be even more impressed if it used Amazon's Mechanical Turk. :)