iPhone 3G - third party apps won't start after reset - issues and workarounds

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.