I was surprised they took up Prop 8. Usually they don't take up such narrowly defined decisions unless they either want to A. overturn them or B. expand them. Why would they bother wasting their time taking up a case that applies, in essence, to one state? If they were interested in keeping the scope the same they probably would have waited for further legal challenges in the same vein.
Well, keep in mind that only four judges need to agree to hear a case (this prevents a majority from dictating the court's docket). So by taking up such a narrowly defined decision, said four might think that it probably limits the potential damage while still allowing the greatest scope for expansion.
I still wouldn't be surprised if Kennedy stops short of actual gay marriage.
And whatever happens, I'm almost looking forward to what is sure to be an amusing (depending on one's sense of humour) Scalia opinion.