Totally agree iPhone 5 should be lighter. The iPhone 4 is great but a tad too heavy in one hand. I think the back glass adds weight and they will switch to aluminum.
If my iPhone were any lighter, I'd have trouble telling when it's in my pocket and when I've forgotten to grab it.
Or 'How to know when you should start working out.'
I thought I would be the first with the same snarky comment :) Seriously though, I use my iPhone 4 in much the same way all the time and have never found it to be too heavy in any way. The solid feel also makes it less "toy like", which really doesn't impact function, but sure makes me feel better about what I spent my money on!