I had problems with both the iPhone 4 phones in our house.
- Purchased within a week of each other in the UK as soon as Tesco Mobile were offering them (within a few weeks of launch).
- Ran fine while in the UK using Tesco Mobile (basically, O2)
- Ran fine from Nov 2012 in Belgium on Proximus
- Problem occurred in Belgium on Proximus around a few weeks ago as others observed
I've just done a factory reset and manual sync on my wife's phone and now working fine from my iPad (3) and vice versa. Biggest pain for us is sorting through the apps and putting them back in folders - not too much of a hardship.
I can also confirm that a full restore from pc or iCloud did not work in my case - needed to be a manual sync from iTunes. This has been reported by others in this thread too - you end up restoring the fault.
I'll be doing mine tomorrow... thanks to those who posted this workaround.