I'm not sure how the law works in the US (the federal/state divide - if divide is the correct word - confuses me).
However, there is an interesting case being considered by the Irish High Court at present. Details can be found here.
Basically two lesbian women, who married in Canada, are seeking to sue the Irish Revenue Commissioners (something similar to the US IRS?) after they returned to Ireland to find that they would continue to pay higher taxes as single people, rather than less money as a married couple. They are claiming that the equality provisions of the Irish Constitution have been violated.
Judgment has been reserved; which basically means that the judge is considering all the legal arguments before making a decision.
The two women have indicated that they are prepared to appeal to the European Court if they lose the case (in the EU a case can normally only be heard in the European Court after it has been through the courts in the member state first).
The Irish Government may well appeal the case to the Irish Supreme Court should it go against them. But, that said, the Government is currently considering legislation to at least recognise civil partnerships (if not full same sex marriages) along the same lines as happened in the UK recently. Many gay activists in Ireland are of the opinion that a Civil Partnership bill would only give gay and lesbian people second class status.
Homosexual acts were only legalised in Ireland as recently as 1993, on foot of a case brought to the European Court by the (now) Irish Senator, David Norris, in 1988.
~ Marty~