NOTE: On 9/30/2006, Congress approved the Unlawful Internet Gaming Enforcement Act of 2006, which changes the answer to the question. Read updates here
Like many legal questions, the answer to the question of is online poker legal is yes. And it's also no. And maybe.
The trouble in answering the question springs from a very simple source; there simply aren't any legal precedents in regards to playing poker online. No person in the US has been charged, brought to trial, convicted, or sentenced for playing poker online. While some interpret the Wire Act as potentially barring online poker, there is absolutely no legal precedent to base that argument on.
Traditionally, gambling statues have been enforced in the US on the state level. Some individual states do have laws on the books that can be interpreted as prohibiting Internet-based gambling. Once again, it should be pointed out that as of the writing of this no person has ever even been charged, much less brought to trial or sentenced for playing poker online. Millions and millions of US citizens play poker and gamble online each and every day. And not a single one of them has ever been charged, fined, or prosecuted.
Based on the above, the odds are overwhelmingly in your favor that you can safely play many, many hands of online poker without any legal repercussions whatsoever.