Interesting... Thanks for the feedback.

First up, no sign up is required to create the bill. Check out the mock-ups: http://blog.split-the-bill.com/split-the-bill-the-mock-ups

Also there are no emails notifications (yet). You are free to IM, tweet, SMS or Facebook the link to the bill.

Of course we are looking at mobile phone apps and an API for other systems to integrate with (How cool would it be for a POS to include a link at the end of your receipt ?).

Integrating a payment method would also be a possibility.

Nonetheless I do like the idea of supporting more formal agreements and larger costs... will add it to the road map :)