Players have little to fear. There is no federal law against playing online. U.S. Attorney Catherine Hanaway admitted in a 2007 House hearing that just placing wagers online isn't against federal law. Players might run afoul of state laws, but even that is rare. The one case we know of is Jeffrey Trauman, of North Dakota, who paid a $500 fine on what was probably over $100,000 in sports bet winnings. (Gambling & the Law)
Rather than just considering the law, it's maybe more useful to look at the potential risk of each activity. That is, which activities are more likely to result in a fine or maybe even jail time? Here's how things stack up, from most risky to least risky:
Most Risk
Taking bets on a server located in the U.S.
Taking sports bets on an offshore server and then visiting the U.S.
Facilitating the transfer of funds to online casinos (payment processors), and then visiting the U.S.
Helping people make bets on a website
Accepting advertising for Internet gambling, in the major media
Taking casino / poker bets from U.S. customers on a website outside the U.S.
Accepting advertising for Internet gambling, smaller media
Buying advertising in a U.S. publication as a casino, poker room, or affiliate
Placing bets yourself on the Internet
Least Risk