Looks like the JB is causing me problems if anything...
Could very well be. I had a cycle of restores and resets last night, firstly the voicemail needed setting up every time I went in, so i fixed that, only to have the lock screen password timeout die (could only be set to "immediately"). Had to restore a blank IOS and set the phone up again from scatch. Both these bugs are present on non-j/b phones though.
Luckily I've only had the phone a week so I didn't lose much apart from a few game saves, and on the iPhone4 the restore is pretty quick.
This time I set up all my settings, made sure voicemail and the lockscreen were working, did a backup, then synced all my media, backed up again, and only then jailbroke and put my essentials on (lockscreen, sbsettings, wififofum).