Doing my first IM at Lake Placid this month after a couple of years of Sprints and Olympics. Nothing too spectacular, but expecting mid 13 hour time. I used the Mark Allen 18 week plan as my training guide and it has worked well I think. I intend to sign up for 2006 IMLP, and I wonder what sort of a plan I should go to afterwards to recover, and maintain the fitness I have achieved over the last six months, prior to getting serious again next Spring.
Any suggestions?
Given that this is your first IM, the biggest variable/unknown will be to see how you feel once it is over. Not sure what your plans are for races later on this Summer, but you will likely feel bad for a while and you will want to take plenty of time to recover and refresh your mind to the point you want to look at your bike again 
You would probably find that you’d feel flay at an Oly a month after. You will have built an impressive base and so it would be stupid to waste it. Go by your own feelings and keep up some exercize once you are “ready” for it, but don’t try to do too much too soon because that might put you off a nice Fall/Winter base period.