Twice in the last two days my iPhone 3G has not started properly (it gets to the white apple logo and no further).
In each case - a reset (press and hold both power and the main button until it stops) has allowed the phone to boot (albeit slowly - 2 mins or so).
But - once booted - it can only run the inbuilt apps - no third party apps run.
Last night I did a recover from backup. Today I did not want to go thru that again - it takes a long time.
Some googling has suggested that the phone loses some kind of authorization for apps. It suggests that adding a new free app or removing and re-adding an app can fix this.
It doesn't work for me over wlan or 3g. It does prompt for the password but the download never starts - and after a restart it just says "Unable to purchase".
Via iTunes and a USB cable it did work - simply removing an app then re-adding it (a sync required for each step) has re-enabled the apps.
I have no idea why the resets were needed (hope it's temporary) but at least this brings all the apps back without a full restore.