You’d think the top search results on the matter would be newer than 1999, but that’s where you’ll find this NYT article and PubLaw item story, both from precambrian times. Worse, both of those articles suggest that my links to them may not be entirely kosher.
The problem is probably that US courts have not spoken clearly on such a case. A Danish court in 2006 did, but I think that no case in the US has gone far enough to actually set a precedent. Another chance at settling this issue was lost earlier this month when BlockShopper settled, rather than continue a costly defense of such a case. The EFF is confident BlockShopper could have won, but that means little when the legal bills come in.
Related at EFF: Kelly v. Arriba Soft and Unlawful Internet Gambling Enforcement Act.