YEESH! I checked out your photos and that is indeed nasty. It's strange, I've never encountered that on my own 24" and I live in an extremely humid country.
Is there any extreme source of moisture in the area you're using your Mac that you could pinpoint (and eradicate)? That just doesn't seem right, especially on four different iMacs. The glass can be removed by the end user. It's only held on by magnets. There are a ton of Youtube videos showing various methods to do so including suction cups and duct tape.
I don't recommend it though because it's extremely easy to get dust particles under the glass once it's removed and they end up resembling stuck pixels that flicker.
Incidentally, you were quite lucky to get Apple to replace your first iMac for a single dead pixel. They usually won't do that unless you have a few at least, in prominent locations.
It happened again tonight when i turned on my iMac to work on my assignment after being at Uni fo over 12 hours ugh!!!!
http://www.flickr.com/photos/23708971@N07/2453618739/
Im sitting at a desk with nothing near moisture.. The thing is the same thing happened when i got my 3rd mac and started it up at my boyfriends place. This was summer and it was cool inside - not humid at all.
Ive read many forums about this problem and also removing the glass to clean it up but yeah, not too fond of removing the glass
Anyways i want to email the lady ive been dealing with but i feel so bad bothering her! She has been nothing but helpful over the last few times...
Im definitely buying AppleCare before my 1 year is up!!!!
Btw i know im really lucky about the dead pixel