It might work for a while but it would need to be more advanced to work. If not, the perps will just set up accounts that retweet each other. It would also have to meassure if the retweeting account has good karma as well, if not, don't count it. But hey, let the arms race begin!
Google has the exact same problem. Surely one could borrow some of their insights. i.e. to detect 'retweet farms'. If all the spammers retweet each other, they'll still be on a PageRank of 1. No matter what you do, there'll be people who try to hack it. Twitter (and all organisations in this problem) can only keep trying to make things better.
