I think this is kind of a big deal, as aside from the occasional sit-com cameo, we haven't really seen MH in a prime time show, in a major role. I'm hoping he'll be playing the baddie, but the details of his character are being kept a secret:

http://ohnotheydidnt.livejournal.com/76832269.html

http://www.tvguide.com/News/Criminal-Minds-Exclusive-Mark-Hamill-Season-Finale-1063845.aspx

I think the timing of this is also interesting, with all the rumors of his being cast in the next Star Wars sequels.