The last few iMac updates where great, with the C2D and CD, but I feel a new enclosure and more features (more RAM) and bigger screen, PLUS, how old the current ones are makes for a more likely case than the previous updates. Just my opinion. If there were a betting table, I'd put my money on the 7th.
LED Displays maybe? They did that with the new MacBook Pros. It does not seem too far-fetched that they would apply the technology to the new iMac as well.