I also recently purchased an iMac 24", and I'm not too concerned. I purchased a refurbished model, and it's absolutely perfect. No gradient issues, no condensation, nothing. Since staying under budget was an issue for me, I won't be in the market for a newer model that would most likely be in the $2000 range anyway.
After the rumors of the Blu-Ray stuff the closest thing to an upgrade I see would be a Blu-Ray read/write option but knowing Apple it would probably be in the idea of at least $500+ for the add-on.
Some others have also speculated about the idea of an upgrade to the video card.
If these two things are of significant interest to you, and you aren't concerned about cost, then I'd wait. If cost is an issue, and these things aren't that big of a deal, then go ahead and buy. 😀