Venmo Review
alexeymk.com
alexeymk.com
I actually did this in college when the federal government allowed people to buy savings bonds with a credit card. Unless they have really good controls, I can see some enterprising college kid pumping 250k through this service and showing off an Amex black card.
(I am a competitor but this is a real question.)
[1] http://geoshare.us/ [2] http://news.ycombinator.com/item?id=2093261
The only difference is a few features and that (for now) they aren't charging fees.