iPhone back to OS3: What happens with application data? I have upgraded my iPhone 3GS to iOS4, but now want to go back to OS 3.1.3 that it had before.
There are plenty of instructions on how to do this, but what happens with the data stored on the iPhone?

I don't mean the data that is usually synch'ed with iTunes. I have several apps that just store information I have entered, with no function to sync it or transfer it to a PC or similar. A simple example would be the saved scores of an installed game.  

If I would have to re-install the iPhone, I could restore a backup which would include the data, but when going back to OS3, I cannot use a backup since it would restore iOS4 also.

Will the installed apps and their data still exist after re-installing OS3?
If not, is there any way to keep the data?
Restoring firmware and restoring settings are two different things. You can restore to a stock version of the 3.1.3 firmware then when iTunes asks you to set up your device, you can select which settings image you want to back up from.

Or select the iPhone under "Devices" in iTunes >> right [or CTRL] click it >> Restore from Backup. Should work.