Here a tip for those of you that have problems updating your iPhone to OS 4:
If your backup of the iPhone takes more then 2 hours when trying to upgrade or even stop with an error message, then you could try this:
- deactivate in iTunes on your iPhone devise most off your apps
- deactivate in iTunes on your iPhone devise all your playlist and song to synchronize
- deactivate in iTunes on your iPhone devise sync to your photos
- click apply (bottom right) and sync
- disconnect you iPhone the normal way
- reconnect you iPhone and run the upgrade to OS 4
- activate in iTunes on your iPhone devise most off your apps
- activate in iTunes on your iPhone devise all your playlist and song to synchronize
- activate in iTunes on your iPhone devise sync to your photos
- sync you iPhone again with iTune
The only problem with this is that you have to rearrange all your icons again. Don't forget that you can build folders now.
This is just a tip. It probably doesn't work for everyone. And you do that on your own risk. But it worked for me.