Thanks for that, Alex! 😊
I wrote a feature about this some time ago ( http://www.allaboutsymbian.com/features/item/The_Last_Smartphone.php ), people think the revolutionary mobiles are stuff like the iPhone and N95 but they aren't. The mobile revolution is about providing communications and trading infrastructure in the poorer majority of the world where little or none exists. We can get all wide-eyed about our latest toys, but technology as simple as the text message is far more important when it starts changing communities and helping people out of poverty. It's the cheap end where the really exciting and world-changing stuff always happens, and phones are no exception.
I think a lot of phone functionality will move onto the web, once enough phones have browsers capable of accessing Google Documents-type sites properly. As you say, ajax would be a very interesting thing to see on a wide range of phones.
It would make sense to store your data online, if you could be reasonably sure it was secure and private, and if you could be fairly sure of getting connections to the internet when you need them. It also wouldn't matter what happened to your phone and you'd never have to synchronise it. Indeed, you could use any ajax-capable device any time you wanted: a phone, a PDA, a desktop computer or any other class of computer that comes along. As long as you can remember your passwords you could do exactly the same things on any of them. The devices would be as interchangeable as tin openers.
If that happens, the OS of the device would become mostly irrelevant as its only major task would be to run the browser and user interface.
It would merge mobile and desktop computing as the computers themselves would just be windows onto the web. Web servers would be doing all the storage and processing, and consumers would just be using terminals: pocket-sized ones called smartphones and desktop-sized ones called PCs.
Very processor-intensive tasks like cutting edge PC games would be difficult or impossible to do on the web in this way, but most people hardly or never use processor-intensive tasks. Most people use computers entirely for things like web browsing, email, word processing and so forth, which aren't at all demanding and could be done with virtually any PC or smartphone in the shops right now.
"unless Symbian become very cheap or open source"
It is pretty cheap, it only costs manufacturers something like a few dollars per phone. And if it's cheaper than Linux for the manufacturers to work with, it may actually save them money to stick with a commercial OS. I think it helps that Symbian isn't controlled by any one company but a number of rival manufacturers, so it's sort of semi-open if you see what I mean.
Linux on phones is doing very well in developing countries though, and I wouldn't be at all surprised if it became the main OS some day.