Actually it is illegal, some people here need a lawyer. If you are running a gambling site in the USA think of leaving the country and change the server location.
There's nothing explicitly outlawing "Bitcoin gambling." However, most states have laws against running a gambling operation and taking a rake. These generally consider gambling to be for "anything of value," and anyone arguing Bitcoin doesn't have any value is not only FOS, but not going to be very convincing to a jury. After all, why would anyone take a cut of something with no value?
The feds have maintained the position that the Wire Act prohibits Internet gambling operations (gambling operations defined as above), but it is far from certain this is actually true. In fact, the recent Unlawful Internet Gambling Enforcement Act (UIGEA), the cause of "Black Friday," only prohibits bank transactions connected to "unlawful gambling" operations. So Bitcoin-only sites don't have to worry about that particular law's banking prohibitions.
So while there's no clear federal law prohibiting online gambling per se, people like Calvin Ayres (Bodog) are fugitives at present as a result of federal claims that there are. On top of that, individual states do have much more clear laws prohibiting "unlawful gambling." If a site is knowingly accepting visitors from a state (or perhaps is merely being willfully blind), that state can and perhaps will exercise jurisdiction under its long-arm statute, as that constitutes "doing business" in the state.
Anyone who is going zip-a-dee-doo-dah and just assuming nothing will ever happen is burying his head in the sand.