By now, it's pretty much assumed that any unencrypted data on your phone could be accessible to someone with the right tools, and with its latest guideline updates, Apple's further confirmed that to be the case where iOS devices are concerned.
Apple admits that if a warrant is issued, it would be able to extract certain data from a locked iOS device. Those running mostly third-party applications will have nothing to worry about, as the data Apple can snag has to be from its own applications. Further, emails will not be able to be extracted, nor can calendar entries.
Due to iOS' design, Apple would be unable to provide law enforcement with historical GPS data, though it does admit that email logs, email contents, and other data backed-up to iCloud can be under certain circumstances.
Here's where things get interesting: According to the company, data can only be extracted from a device at its Cupertino HQ. Why this is the case, we're not sure, though it seems likely that there's some seriously low-level security functionality in iOS devices that we're unaware of. Further, it could be that in order to handshake with this extracting ability properly, the GPS location must match Apple's HQ. Or, I could be totally off-the-wall. Either way, it definitely has me intrigued.
As always, let this be a reminder that not all of your data is safe just because you use a lock screen. At the very least, be sure to use encryption when possible, because even if data is taken from the device, it's likely to be useless to anyone who has it.