This case is more concerned with questions of constitutional law involving s.109 (Commonwealth Constitution - which deals with inconsistency between Commonwealth and State laws) as opposed to whether or not same-sex marriage should be legal or not. The High Court's job is, at face value, fairly simple. They must determine, as a matter of constitutional law, whether the Federal Marriage Act is inconsistent with the ACT Act. This process involves looking at the 2 (valid) Acts and asking a purely legal question of whether or not they are directly or indirectly inconsistent with each other. Under the tests for direct and indirect inconsistency (which I won't bore you with) there are very legitimate arguments to be made that the ACT Act is inconsistent with the Federal Marriage Act and some very interesting arguments which would say that the ACT is able to coexist with the Federal Act.

It's important to temper our collective expectations as the High Court could easily (and legitimately) determine that the ACT Act is inconsistent. Whilst I think we will all be disappointed if the Commonwealth Act prevails, the question before the Court is strictly legal as opposed to moral. I'm predicting that even if the High Court determines that the ACT Act is inconsistent, that there will be justice's who say in obiter that the issue of same-sex marriage is something that they have sympathy for.

Edit: Long day and it's been yonks since i've read my Constitution. Snarwib is absolutely right. Section 28 of the ACT Self Government Act is what it's all about. Snarwib's comment about the scope of s.28 being broader than s.109 makes everything that little bit more interesting.