I'm in a similar situation.
I go crazy at my PC at the moment as its so terrible and I need a replacement, the iMac will be it.
I want to buy ideally between now and June, so its all a gamble as to whether they do do an update around June for Montevina or not, thats what it all comes down to.
If I get a Penryn iMac in June, I'd far rather have it now if its the same machine (now as in the speculated soon update), but wether its worth waiting depends on how likely another update in June is.
I'm in two minds as it would be very soon after another update, but yet people here seem convinced that the iMac must become based on the Montevina processors as soon as they come out (I don't know why, but assume there is some reasoning behind that). Plus the 10th anniversary in early august and an apparently book WWDC in June all suggest its possible.
I really really would want the iMac as soon as its updated if its the same one I would get in summer, but would hate to get one now only to have it updated in summer anyway.
However, unless the current one provides a significant update, which I doubt, I expect I'll wait to be on the safe side, and just live with my pathetic excuse for a PC.