^or have the CPU's heatsink be the screen's base plate, which means the CPU heats the screen up, well at least it isn't sub zero Celsius 
... That would make it worse. In the situation described, the screen cracks because it does not shrink as much as the aluminum does. Heating up the screen would make it expand, which would further accelerate the cracking.
Speaking of innovation, you seen the Lumia 920? It looks like it might be something of a dark horse that nobody saw coming in this one. If Metro pays off, they actually have been busy working on some nice features; a curved screen for better viewing angles, particularly in sunlight? Night-time photography? Augmented Reality? New tech in the screen that decreases the refresh speed so it doesn't blur when you move it? Not to mention that it's reputedly built like the old school nokia phones...
You mean increases the refresh speed. But man I can't wait to actually hold it in my hands. I found the lack of a microSD slot and the fewer color choices compared to the 820 to be a bit of a turn-off though.
I'm also particularly interested in Nokia City View. That's the coolest thing I've seen so far. Wireless charging is a neat feature too.
As for build, I don't think I've seen any Nokia phone that was fragile. In the smartphone line I've handled the N8 and the X7. Both feel pretty damn solid. In the semi-smartphone line, I've handled the C6-00. It was alright, didn't feel quite as solid but would probably still break the ground. In the regular phone line I've handled the 5300, 5310, 5130, 6301, and one other model that escapes me atm. All of them except the 5300 felt totally solid. The 5300 I'm not too sure about, since I managed to damage the screen within the first month.
I believe lumia 920 is a windows phone like lumia 800 was.
I have seen lumia 800 interface and it was truly unimpressive. I'll have a look at it later on.
I currently have an iPhone 4 and I am not gonna change it for another 2 years atleast. It's an year old already.
The 920 and 820 are built for Windows Phone 8, yes. The old Lumias used Windows Phone 7, which was largely unpopular for some reason. Since Windows 8 has a new interface that works best for tablets and smartphones, I'm pretty eager to see it in action.