http://en.wikipedia.org/wiki/UIGEA
UIGEA "prohibits gambling businesses from knowingly accepting payments in connection with the participation of another person in a bet or wager that involves the use of the Internet and that is unlawful under any federal or state law." but does not make it a crime for a citizen to engage in the activity.
http://www.gambling-law-us.com/State-Law-Summary/
You are right though, the UIGEA makes it illegal to transfer money from a US citizens account to an 'illegal gambling operation.' It doesn't actually define what an illegal gambling operation is.
I'm not 100% sure as it's been a while since I've kept up with this but I believe that Poker has always been in a sort of gray area. I think there is suppose to be a distinction between games of chance and games of skill. In the past only online sports books or true online casinos had ever been prosecuted in the US.