True. The old G5 17" iMac I had bought my folks 2 years ago was awfully bright out of the box, too. But, that isn't exactly the problem here. What you're probably experiencing is the result of the LCD film type Apple has decided to use on their new 20" iMac. If you look at the specs, it only has a viewing angle of 160 degrees which is even worse than the previous 20" model which had a 170 degree viewing angle.
I live in Boon De'Ville (aka: Myrtle Beach), South Carolina and we don't have an Apple Store, so I can't support my opinion yet. But if Apple is using TN film in their new line of iMac LCDs as oppose to IPS film like their Cinema Displays, then you among many others are in a world of disappointment as I once was/am.
If you're a designer or photographer, you'll notice the following…
Looking at your iMac head on, does your display appear darker at top, and washed out at the bottom? Use a solid color for your desktop image. Does it look like a gradient? If so, then most likely the LCD is a TN film-based matrix.
Because of the cheap parts, TN LCDs are produced alot these days. They are total crap and they are everywhere, like Circuit City, Best Buy, Costco, etc. It would be very hard to find an IPS or PVA LCD monitor, unless you go to a specialized tech store.
People need to be educated about this issue. It's serious. Coming from a much older LG (a beautiful IPS LCD), I never expected LCD displays to head in the wrong direction.
According to the specs, I'm guessing the new 24" iMac does not have this fault, since TN film can not display a 178 degree viewing angle. So, I am guessing the 24" iMac uses the same IPS film as their Cinema Display.
My suggestion would be, go to the Apple Store and see for yourself. Kneel down in front of it as if you were viewing it from your desk chair, so you can get a good idea.
For more info on LCD types, do a Google search. This topic is all over. Also check this link out:
http://www.tftcentral.co.uk/articles/whichscreen1.htm