A handful of tweets from WA iPhone owners seem to indicate that the problem is only effecting Optus customers. We asked Optus about the issue and will update when we get a response.
UPDATE: The problem’s been fixed, according to this Optus statement:
“Optus can confirm that we have experienced a minor issue with automatic time setting, due to an hardware upgrade which was undertaken this morning in West Australia.
“The problem has been rectified, and customers still experiencing problems are encouraged to turn their phone on and off and the automatic time setting should be accurate.
“We apologise for any inconvenience this may have caused our customers”.