Ahhhhh, PIDS (Post Ironman Depression Syndrome)! So common. In fact, I think you'd be hard pressed to find anyone who does not experience this. You've trained hundreds of hours, have had support from every sphere of your life (Family, Friends, Co-Workers, etc.), you've built up to a huge day of racing and you then experienced an incredible day with thousands of people along the course cheering you on. Then, within 48 hours, the banners are down, Ironman Village is turned back into a vacant skating oval and the "Welcome Can-Am" signs are up, welcoming the "next" group to come into town. What's not to get depressed about?
What my suggestion is: Go back and relive your day. Think about all of the great (and all of the painful) moments of the day and what you were experiencing at the moment. "Revel in the Glory" so to speak. You deserve it. That will help. Tell your friends about your experience, but try not to kill them off with hours of Ironman tales. And finally, write your game plan for next year: what were the strengths and weaknesses of this year's training and racing? What would you change for next year? What specific weakness would you spend time developing? It doesn't matter if you're doing another IM in 2012 or not. You're likely to be planning SOME event, so make plans around that. I think you'll find that the internal (but written) debrief process and forward thinking will get you out of the depression and into an inspired state, motivated to move on to an even better 2012.
At least, that's what works for me and what I suggest to my athletes!