This has been fixed with a firmware update for quite a while now.
I've bought a 27" iMac i5 several weeks ago, and it never showed that problem even without the firmware update. But I must add that I purchased a computer from the show room, so I was 100% sure that the display was perfectly in order.
Anyway, I think it's generally perfectly safe to purchase a 27" iMac now. In doubt, go to a store, ask for a demonstration and purchase exactly the computer that they showed you.
Of course, you can also wait for a refresh - which nobody can tell you when that might be and what problems the next generation of machines might have.