I took a quick look at the 5.16.10.0.1 firmware and found a few undocumented changes.
- New annotation types: asterisk, circle, and underline.
- Optional automatic updating of GoodReads (auto-shelving) upon book completion.
- Transition to a new method of handling annotations with them being stored in both the old way and the new way temporarily.
- Internal changes related to inter-process communication.
Many changes need to be enabled remotely be Amazon so the new features may not be made available for some time, possibly never.