iPhone 3G users who've upgraded to iOS 4 are discovering that the roaming switch isn't working any more, for those on O2's network at least.

Apple's new OS isn't running as smoothly as intended; users report the upgrade causes slow-downs and freezes, not to mention draining the battery and running up unexpected roaming bills, all of which should be keeping the Cupertino engineers busy for a while.

Apple confirmed, to the Wall Street Journal, that it is investigating reports that some users upgrading their older iPhones to iOS 4 have seen the life sucked from the beloved handsets as the new OS increases the energy drain while slowing down the interface.

Meanwhile several Reg readers have been in touch to report that their upgraded handsets have run up considerable bills while travelling, despite the fact that data connections when roaming were switched firmly off.

All the complaints we've heard, and seen posted on the Apple support forums, are from O2 customers, and the operator told us that it's aware of the problem and is investigating the issue in conjunction with Apple.

Whether the problem is restricted to O2's users, or if it's just because there are more iPhone users on the O2 network (or even down to O2 having more customers in the jet set) we've not been able to establish, but if you've upgraded to iOS 4 then take care when travelling abroad.

Luckily, for all its faults, the new OS does include a function to switch off cellular data manually, rather than relying on the phone to detect when it's on a foreign network, which deals with that problem. Still, if your handset is overheating, freezing or just generally running slowly then you'll have to wait for Apple to investigate. ®