Startup Idea : Bill splitting for roommates

Bill splitting can sometimes be a complex task. How much is the bill this month? Wait, what percentage of water do I pay? Why not simplify the process? With a function similar to Mint.com which allows you to link up your utilities and any relevant bills. Thus, when all bills come in, an email is sent to all roommates. Could be useful in other contexts, like a fraternity house or other group living situation where the roommates change. You could improve the process by allowing everyone to pay their portion online and avoid the whole (who wants to write the big check issue). This concept could be applied in a variety of situations and could be developed into an iphone app for paying for a trip.