Are the changes to Lion's screen sharing documented anywhere publicly?
Maybe … multi-session screen sharing — the license for Mac OS X (Lion) uses the expression separate graphical desktop session.
Mac OS X v10.6 to v10.7 API Diffs
Items of interest but nothing overtly related to screen sharing.
Open source
Released for 10.7 but some of what's there (example: Open Directory) is not amongst Apple's list of open source projects, so I would not treat the latter as comprehensive.
Again I can't see anything specific to screen sharing in Lion.
Timeline of recent documents in the Mac OS X Developer Library
http://developer.apple.com/library/mac/navigation/
Filtering the list of documents for the word screen doesn't reveal anything obviously relevant, but you might like to review that list occasionally.
Overall
Considering what's in the license, my hunch is that Apple will not be especially open about recent and forthcoming changes (if any) to screen sharing in Lion.
Update
Comments from bmike rung a bell. Apple's article HT4715 About Apple Remote Desktop v3.5 includes a section Lion screen sharing with the following information:

A third party VNC viewer will always be connected to the login window. If the login window is not on the display, a new login window is started that is not shown on the display. The screen sharing user can then log in with any valid account on that computer.

If a third party VNC viewer has difficulty, then maybe it's not following best practice, or the most appropriate standard … whatever those practices/standards may be! With apologies for teaching grandmothers to suck eggs (bmike won't need this but someone else might): http://en.wikipedia.org/wiki/Virtual_Network_Computing