VNC client like x2vnc

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

VNC is an interesting beast on OS X. In Lion, OS X finally gained the possibility of having truly virtual sessions, meaning that they aren't displayed on the console. This is accomplished by enabling VNC, and logging in as a user that is not logged in at the console. That new user will have a private session to work in, and the console user will go uninterrupted.

If you're unsure of what I mean by "console", the console is the one and only session that a user directly attached to the computer via keyboard/mouse/monitor can use. That is commonly referred to as the "console" or "physical" session.

Note that on all previous versions of OS X, logging in as an allowed user could only put you on the console session, possibly colliding with a console user if they were actively working on the system at the same time.

On any other operating system, you can set up a private VNC session using almost any resolution you want, within a certain realm of feasibility. This is unfortunately yet to be the case on OS X. VNC still seems to hook into the physical capabilities of the system, perhaps this happens because the video card is used to render the desktop session even when you're using it remotely. Rendering the OS X animations is very intensive, so offloading them to the video card and freeing up general CPU time is an ideal situation for pretty much anybody.

So the answer? I don't believe you can. You can turn it down via the System Preferences like usual, but the highest resolution you'll get is that which is supported by your video card.