Let me introduce you to a concept, I like to call it Rev. A. The new iMacs are a Rev. A item. Rev. A's are pretty much beta testers, although they might not know it. Some people think this idea is B.S. but I can't tell you how many times I've seen it proven to be true. It's always a bad idea to get the first of anything new when it comes to technology. It's even true with cars. I'm just telling you from experience, and casting the Apple "fanboy-ism" aside.
Most of the "bugs" and issues, from what I've read have been ironed out at this point with the new iMacs (except that crappy graphics card and its drivers...it's been months and people are still waiting on fixes), so if you can't hold on you should be fine with buying now (well...at least wait till october 26th when Leopard comes out). However, if it were my money, I'd wait for the Rev. B iMacs.
I'll bet you money the Rev. B iMacs have far better screens (along with all the other minor upgrades that come with each Rev.).
...and of course Apple will be happy to share the revision level information with us won't they? Yeah right!
If there is a major problem with your "Rev A" machine then you just send it back under warranty. In the UK at least, under the sale of good act, if there is a problem within the first 6 months then the onus is on the manufacturer to "prove" that it is not a manufacturing defect. After 6 months that onus falls to the customer. So basically, make sure you are happy within the first 6 months and send it back if you are not.
Cheers,
Craig.