I am very sorry to say, but if new imac's should arrive in time with the ML release they had to be already be shipped..
I am pessimistic about the refresh as well, but you can dismiss your concern on this particular point. With most refreshes, only a small number of base config units are available in store right away, well within Apple's shipping capabilities. ALL CTOs and most general availability is by shipping starting after the fact. I have bought multiple Macs upon refresh and this has consistently been the case. For notebooks, they tend to drop ship straight from Shenzhen to the buyer; for desktops, it varies by CTO option but can come from Shenzhen or from within the United States. There is also a slight delay even for the initial rush of purchases; if we knew for a fact, for example, that the 2012 iMac was due 7/25, a buyer who wanted a CTO should not expect to have it in hand before ~8/3 or thereabouts.
The refresh may still not occur until 2013 -- or never! -- but we won't be able to deduce anything from any lack of shipping prior to the refresh date. Obviously the converse is another story; if an Apple store gets a gigantic pallet of boxes, even if they keep them wrapped up, whichever employees are allowed in that part of the stockroom (and are under soul-crushing NDAs no doubt) will easily figure out what they are. Of course, the public may or may not learn the truth before the rollout.
As of today, I handicap the iMac outcomes approximately:
1:1 - Refresh in 2013 or later
3:1 - No future updates; product end-of-lifed
7:1 - Update 7/25
7:1 - Update 9/19 (the day after back-to-school offer ends)
9999:1 - Update another date between 7/26 and 12/31/2012
Total 50%+25%+12.5%+12.5%+0.00001%=~100%
These figures rectally sourced but based on a careful assessment of the scant existing public information and Apple's past and present tendencies.