Now, that’s a cool idea.
The important thing about BillMonk‘s execution of this idea is that they kept it simple. And they definitely have to keep it simple because I can’t imagine using anything complicated to keep track of a $5 burrito with extra sour cream.
As Michael Arrington wrote, the SMS thing is definitely cool, and I think important in keeping the site usable.
Now, they just have to be integrated with (or bought by) PayPal, and I’ll use it!