By way of background, decisions of the 9th circuit are technically only binding upon the 9th circuit. Other courts and circuits may, at their discretion, give some, a good bit, or a great deal of weight to decisions in other circuits. However, since the 9th is arguably the most liberal circuit, other, more conservative, circuits could well ignore the ruling.
Cases in United States Courts of Appeals are heard by a three-judge panel. A majority of the active circuit judges may decide to hear or rehear a case en banc. Parties may suggest an en banc hearing to the judges, but have no right to it. Federal law states en banc proceedings are disfavored but may be ordered in order to maintain uniformity of decisions within the circuit or if the issue is exceptionally important. Fed. R. App. P. 35(a). Each court of appeals also has particular rules regarding en banc proceedings. Only a court sitting en banc or the Supreme Court of the United States can overrule a prior decision in that circuit; in other words, one panel cannot overrule another.
If the 9th circuit refuses an en banc proceeding, the plaintiff/appellant may appeal to the Supreme Court, just as he or she could do after losing at the en banc level.
The Supreme Court does not have to hear an appeal. If they refuse to hear an appeal the decision at the next lower court level stands as written.
Assume for a moment that the court en banc upholds the appeals court panel ruling. A rejected appeal to the Supreme Court would rule the ban unconstitutional but only for the Ninth Circuit, which includes, besides California: Alaska, Hawaii, Oregon, Washington state, Montana, Idaho, Nevada, and Arizona.
The decision would apply only in those states, and would most likely end up making unconstitutional similar referenda in one or more of those states.
If the en banc ruling reversed the ruling at the panel level, then the opponents of prop 8 would become the appellants at the Supreme Court level. If the high court refused to hear their appeal that would result in the upholding of Prop 8.
This is a big case with big issues. My guess is that the appellants (those who want Prop 8 upheld) will appeal directly to the Supreme Court rather than waste time on what is in all likelihood another loss even if the 9th circuit agrees to her the case en banc.
The Supreme Court would be in a touchy position. They could deny the appeal, leaving the 9th Circuit alone in holding that Prop 8 is unconstitutional. If they take the case, since their court calendar is already pretty full for the court year, which began on the first Monday in October last, they would have to do some shifting.
They might decide to leave the matter until after the election. I think they will decide to hear the case and announce their decision in late summer, early fall.
It's going to be an interesting few months.