Why are there so many apps for an exotic environment such as iPhone? Because developers can make money from it. I don't know how it has been done in Android, but as the world leader of advertisement, I guess Google has put some thought into it.

I can't think of any. Maybe if my car and phone would both be MeeGo and there was some sort of integration, but what good would that do? My car already has an iPod dock and USB, the handsfree works via Bluetooth on any kind of phone. It doesn't have an integrated GPS navigation, but if it did, what kind of cell phone integration would it benefit from except for online access via bluetooth?

I can't see a reason for cell phone manufacturers. For other devices, perhaps if the price, customizability, hardware requirements etc are fine. But then again, Linux is already free. I guess android is equally fit for that and that WP7 will bring MS back into the game.

If it is interesting to the masses and can be efficiently operator locked.

I have hard time believing that. Why not just any Linux. Take a look at Asus EEE Pc. It's hardly larger than a cell phone, it has 1.6ghz cpu, bigger display, runs normal linux distros and gets 3-7h of battery life. My guess is that these devices are getting powerful enough to require little more than a customized gui on top of a desktop OS.

Of course.