One thought I had might be to use some of your ad revenue as a prize pool each week/month/etc. That way people could play with the potential to earn some real money. Hopefully it snowballs so that you can offer bigger prize pools attracting more traffic in a happy money making circle.
I think this could be a really nice niche to be in as you could end up as the "default" free poker site to play at. Why play somewhere else when you could potentially earn real money playing free poker?
I guess the part that makes me think it would be OK would be the lack of "consideration" involved in playing (You're not required to pay or buy anything to play and there's no potential to lose money).
Just for reference: http://www.ago.state.co.us/FAQ/illegal_gambling_FAQ.cfm.html
"For example, if the consideration component is eliminated and no fee, buy-in or other money is required or solicited from the participants in a poker tournament, then prizes may be awarded to the player(s) who perform well in the tournament."
IANAL so take any of this with a salt mine worth of salt :)