A proxy card that is actually backed by various real credit cards. Automatically switching your balance across the optimum set of deals? This is kind of like what Simple.com do, except with credit.

That sounds pretty much like what most mobile wallet look like they're going to offer, and I can't really imagine how it would work (or what the real advantage would be) for non-mobile ones - such as the hassle of deciding in advance which card you're using for which transaction.

BTW, from what I've read of Bank Simple, this isn't what their model is. From their FAQ - "Can I use Simple with my current bank? No. Through your relationship with Simple, your money is held in an FDIC-insured account at one of our bank partners."