The challenges are legal in nature, and have a lot to do with the political strength of banks in the US. Technically there are a lot of companies that could provide banking services tomorrow, PayPal is one of them.

Given the current economic climate, and the recent criminal incompetence of the banking industry, now may be the time for alternatives to step up to the plate. It'd be pretty dangerous, even with banks on the ropes, to go up against them, though. I'd be shocked if even a PayPal or Google or Amazon could actually pull it off, though all seem to have aspirations and current products that bump up against bank services.