Since I have researched this pretty throughly over the past few years....I will give you the answers (thoughts) I have found.
Is Online Poker illegal in the United States
Easy answer YES it is -- No less of an authority than the Department of Justice of the United States has said so many times in speeches and interviews. No grey area, no question..they have said it is ILLEGAL PERIOD end of sentence in all of the US.
Now does that mean it really is ILLEGAL? well not necessarily....they would have to take a test case by actually arresting someone and seeing if the could obtain a conviction and it hold up in appeals. Many lawyers do not beleive a conviction would hold...but none are guaranteeing it
Most likely there will not be any arrests--but that was not the question
Now--to make it more complicated, at least 12 States make it expressly (or in some legalesse form) illegal to play poker. The most well-known is Washington State that makes it a felony. Another strongly worded law exists in Louisiana. Of course, if you have been following the news, Kentucky has also become famous (they are even trying to take many sites domain names away from them--luckily recently slowed down by appeals court). The laws in those states are the reasons Microgaming had restricted the 12 states players from playing, until they pulled out from registering any new US players just recently.
New Administration
Now, while everyone thinks Obama and his allies are on poker's side, nothing has yet been done to change the current illegality, ALTHOUGH, Barney Frank and some other Dems are attempting to at least get the UIGEA overturned as well as possibly making Online Poker expressly LEGAL.
We will have to wait and see if this will happen.
I assure you one thing...if online poker were not illegal in the United States, all those poker sites would not exclude US players and Party Poker would not have paid a big sum to the US Govt for illegal activities