Some forensic tools like Lantern 2.0 allow the use of a passcode bypass file to get around the passcode lock on iOS 4.2 and later. This means that if someone has your device they can get around the normal restriction to not sync until the device is unlocked.
I'm wondering which file or files iTunes creates and stores to convince a locked iOS device to sync over USB. Even better would be an analysis of the specific keychain items are needed to bypass the passcode lock for iOS.
Best Answer
The complete description of the feature on the Lantern site is "Pass code bypass with certificate file from syncing computer" so I don't think "until the device is unlocked" is the accurate way to phrase it. It seems that Lantern re-uses the pre-existing credentials set up between the computer and the phone.
edit: And in re-reading your question it seems you may be aware of this. A quick perusal of the iOS developer documentation doesn't yield anything to me but you can likely determine this for yourself if you have a mac you sync your phone to.
from a terminal window run the command
sudo fs_usage -wf filesys | grep -i itunesand you should be able to see what files iTunes consults when you connect your passcode-locked phone.