Regulators have not yet really figured out how to deal with Bitcoin, so things can change in the future.
It should be noted, however, that a Federal judge in Texas has ruled that Bitcoin is a legal currency. This came about when a guy set up a Bitcoin ponzi scheme and then tried to claim that he couldn't be persecuted because Bitcoin isn't a real currency. Fortunately, the courts disagreed.
Another way to think of it is this: When you exchange for Bitcoin, you are basically buying Bitcoins. Generally speaking, you are only forbidden from buying things that are expressively forbidden by law (i.e. drugs).
Right now, there is no government regulation that says that you cannot buy Bitcoin. And most likely for the government to set up a regulation against buying Bitcoins, it would have to prove that Bitcoins are harming the generally public (as is their argument in regards to drugs).
Personally, I doubt the government could make that case, and even if it wanted to, there are far more important things on their books.
If and when the government gets serious about regulating Bitcoin transactions, it will most likely focus on drug purchases, money laundering, and things that are illegal regardless of the currency used.