I'm on my 3rd G4Ti lap-top, and never had a problem. My boss bought a 400, when they first came out. No heat problem, no screen problem.
My buddy bought a 500 a year later, and it's still rockin.
I bought a 667, and the screen did die on it, when some idiot knocked it out of an overhead bin during a flight. The computer works great, and is now a desktop machine with an external monitor.
The 1 Ghz sitting in front of me is the last Ti model (they went to aluminium after that), and the screen is fine. 7 friends have the aluminium skin, There's 2 12", 4 15", and a 17". None of them have screen/heat problems.

If you are still concerned about the screen, follow a few simple rules.
Don't rap it in blankets,
Don't use it on the stove,
Don't use it at the pool in Vegas
and my statistics prove, above all,
Don't put it in an overhead bin.