Misha gave one whopper of a spoiler to TVOvermind: Cas does in fact get out of Purgatory! Yes, he will be making his way back to earth. Being the cagey guy he is, Misha didn't reveal exactly how he gets out, but now we know Dean will be having what might be a very awkward reunion with his angel friend sometime soon.
When Cas does escape, he'll be doing something he hasn't really done before: hang out with Sam. Despite their rocky history, Sam will be thrilled when he sees Cas again:
"I think when Sam was actively hunting, I think he and Cas never saw eye-to-eye and Cas understandably had some resentment for Sam. I mean, Sam did canoodle with the very thing that Castiel hates the most…with demons. So Cas, being the noble type that he is, always held that against Sam. And Sam is a smart guy, he gets that. I feel like now, Sam…Sam’s had a tendency to try and take the higher road, [which] sometimes put himself in a bad situation like 'good intentions pave the road to hell', but now he’s just happy to see Cas."
We should learn a lot more in the Cas-centric Episodes 7 and 8, but meanwhile Supernatural is all-new next week on October 24.