It's a gray area.
While there is the Unlawful Internet Gambling Enforcement Act, it doesn't explicitly mention virtual currencies like Bitcoin.
The government has a history of interpreting laws, especially when it comes to law enforcement, to get what they want.
If I were making a business out of this, I would be cautious and anticipate that it will likely get regulated in the near future.
NPR had a whole episode on this: Is Online Gambling Legal If Bitcoins, Not Dollars, Are At Stake?