So, if this is indeed the case, how many times would I have to get my panel replaced before I get a non-defective display?
I've no idea.
The one 2009 24" iMac I've seen had this problem pretty severely, quite a bit worse than the Dell 2209wa I have (which itself is just on the borderline of being bad enough to ask for a swap under warranty, or to not bother). The previous 24" iMacs I'd seen were all much more uniform (not a huge sampling of them, 2 or 3).
A 23" Apple Cinema Display I looked at also showed the darkening around the edges, but not as much as my 2209wa and not nearly as much as the 2009 24" iMac.
I wonder what's changed in the design or the manufacture to introduce this drop in quality?