There's not really any way to know. You'll have to keep an eye on the various boards (here, the Apple support forums, etc) to see if anyone mentions a hardware change. That said, what issues are you concerned about in the new iMacs? Compared to other new machines, this one seems to have less reported issues than most.
There's been some trouble with the 24" screens/backlights giving a funny tint, but I think that's been an issue with large LCDs for a long time.
There's the TN panel in the 20" which may or may not change, it's a "design flaw" not a bug.
Some scattered reports of overheating due to various faulty components, but that's not likely to change with an updated machine (you'll still get bad drives, etc).
There's what sounds like a graphics driver bug relating to iTunes, but that's a software issue.
What are you waiting for them to fix? I'm not trying to say the new iMac is flawless, just that I've not seen any substantial reports of a particular issue (like the RevA G5's were super loud, etc). I'd be curious to hear your concerns are as I am planning on ordering one in the next couple days.