One of the reason I decided to leave my Windows Mobile behind was that it was crashing too much, or was low in memory so that for instance you would press the ANSWER button and it would react only 20 seconds later (just when the call goes to the voice mail ! ;-)
iPhone 3G on the other hand has a different aproacg : yes you can now install third part applications (called Apps in the iTunes Store), but NO they cannot run in the background.
It means that if you press the MENU button (only button on the iPhone.. to go back to the home screen), then the app that was running will stop, and you can start another one.
Saying that this is not really true if we consider the other applications that are still running at any time : SMS, Mail, Calendar, iPod ... but all of these .. coincidentaly.. are Apple owned applications.
So basically as an end-user who consider himself "advanced user" (since I test and test all the apps I can find) I was a bit disappointed that I could not keep my Last.FM app running to listen to music WHILE checking my Mail application (as Last.fm would stop when I go back to Home to open Mail ... got it ?)
Specially more disappointed that at the if you check the video of the annoucement of the iPhone 3G in June, Steve Jobs and his team announced the great feature they came up to provide a better performance of running multiple apps on the iPhone : ie by NOT running the apps, but letting them display "notifications". So far there is none :-( (one of the only notif are the mail but even so ,it is download and THEN notified ot the user)
But it's coming .... read the article on the link below .. it's coming as developers have been provided with the API to use them.
Am REALLY looking forward to play with the first app using notification !
Push Notification Service: Apple's Solution to run iPhone Apps in the background - iPhone Hacks