I upgraded to an iPhone 3G recently and love it. I am doing this one legally and have not Jailbroken it so I dont have all the cool 3rd party apps from Installer App.
The main app I miss is an IM service app. I used to use Fring but now can't so I was delighted when I found Palringo. Very nice app but due to the restrictions on iphone apps, it cant run in the background.
This morning, a new chat app called MobileChat from Twenty08 was released which promises support built in for Apples Push technology. This will enable the app to notify you of IM messages without having the app running in the background.
I see the Push technology being a major benefit for the iphone with the current app restrictions. The main reason you would want an app running in the background is to have it "listen" for events and alert the user.
If the Push technology (promised to be available to all SDK developers) does what it intends, this would be the perfect solution and will solve alot of issues for alot of existing great apps.