I think sending an iMac back for a single stuck pixel was a bad move on your part mate, not meaning to rub it in or anything. Stuck pixels can very often be unstuck, and there's even a Dashboard Widget for OS X that rapidly flashes red/green/blue for "unsticking" stuck pixels.
Everyone, quite rightly, wants to receive a perfect screen but the nature of the technology is that stuck or dead pixels are reality that exist regardless of how much care is taken. Apple merely purchase the display panels and send them to be put into their product, meaning they have even less control over the dead pixel situation.
As it turns out you send back a stuck pixel and ended up with more stuck and even a dead pixel. Personally, I'd have persevered with the stuck pixel until it eventually unstuck, even using the widget I mentioned to help it on it's way.
Display manufacturers have thresholds, many of which will point blank refuse to exchange a display with stuck or dead pixels within "reasonable" limits. Sony are the worst for this, as noted with not only their Bravia series but the PSP's screen also. Unless it's like snow, they don't want to know.