At the proper usable distance of an iMac it wouldn't take much of a boost to get it into the "retina display" range.
Personally I don't think it needs it. I need to update my 24" iMac anyway so anything is a bonus.
The first iPhone was unveiled on January 9, 2007. iPhone5.com was registered 13 months later so Apple didn't happen to use a copyrighted name that this guy already owned the domain for, it was the other way around. It was probably just the lowest available number after the word iphone. The fact that Apple didn't register it would tell me that they aren't going to call it iphone 5. However, it's still a copyrighted name so unless it was registered before the announcement...
I'm pretty sure that Apple went for this because of the shorter lead time. I can't imagine the Apple R&D guys all huddled around a touch screen complaining about how thick it is. They'll easily lose more than that by using Gorilla glass 2 on the front and liquidmetal on the back.