Geoff & everybuddy,

There was a "sort of" federal case some years ago, as I recall.  I think it was a case in Minnesota in the 1970's where a gay couple brought a case all the way to the Minnesota Supreme Court, where the state justices ruled that same-sex couples did not have a right to marry under the U.S. Supreme Court ruling in Loving v. Virginia (the landmark case that struck down laws prohibiting people of different races to marry).  As the Minnesota Supreme Court ruled using the precedent of a case (Loving) based on the U.S. Constitution, appellants were able to appeal to the U.S. Supreme Court, who in turn dismissed the case for want of a "substantial" federal question.  By letting a lower court ruling stand re: a federal question, even absent an official opinion by the U.S. Supreme Court, this established a limited precedent for all lower federal courts to follow. 

In other words, since the Supremes didn't say that the Minnesota Supreme Court was wrong in it's interpretation of Loving; that same-sex couples don't have a right to marry (as interracial couples do), it is reasonable to infer that lower courts are required to follow the high court and do the same.  Hence, a "sort of" federal addressing of the issue.

However, in light of more recent court rulings (e.g., Romer v. Evans and Lawrence v. Texas) -- the High Court might feel a more "substantial" federal question(s) has arisen and the issue is more ripe for adjudication.