If you buy an iMac now you can get a free upgrade to ML just by asking within 30 days of purchase. I ordered a i7 with a 3.4 Ghz processor and the 6970M and it took four days to ship. Should have it Saturday...
If for whatever reason new iMacs don't release when ML comes out, it should take no more than 5 days to be able to buy a 2011 iMac with ML preinstalled.
Apparently not Apple. I thought they were 30 days....I think they will honor a 30 day return if the product has a defect because I am sure I had returned an imac before that was just under 30 days....but you know, memory is the first to go!