You cannot make your currency resemble official US currency in any way that would confuse a normal citizen. This has been interpreted rather broadly. The Secret Service has shut down a number of private currencies due to this rule, including silver/gold backed paper money and coins, Liberty Dollar, eGold, etc. I think they shut down the guy that was putting a bitcoin sticker on gold coins as well.

The reason they have not shut down Bitcoin is that they do not consider it a currency. It isn't money, it is a commodity at best.