So you're saying that Disney dollars, Facebook credits, Wow gold, and frequent flyer points are all illegal in the US because they can be used to buy something and don't have a US dollar value?

Bitcoins in and of themselves are almost certainly without a defined legal status at this point in time - after all, they're essentially just an electronic token and there are literally thousands of different types of those in use.  What might be illegal in some jurisdictions are some of the services which are associated with Bitcoin, but until some attempt at prosecution is made and a court rules on the matter, then little can be presumed.  The law is about more than just statutes - it's also about precedent.

One area where Bitcoins may come under scrutiny soon is online gambling.  The US Department of Justice has already taken action against online gambling sites and third party payment processors for accepting funds from US citizens for online gambling.  It's unlikely that sites attempting to use Bitcoin to get around the US laws related to online gambling will succeed in claiming that they aren't "accepting funds for real money play".