In Apple OSX 10.5 Leopard there were some hacks to allow ScreenSharing to have more options, specifically a Bonjour Browser and fit to window. Basically they used Apple Remote Desktop.
In 10.6 Snow Leopard these options are gone.
Why do I need VNC? Well I have a single PPC only application that I cannot get rid of yet. I considered running 10.6 under VirtualBox but of course this is not allowed by the licence, and it seems it also has performance issues.
I used CotVNC back in the day, before Apple's own Screen Sharing became useable. I'm back on
So thats my problem solved: I leave one machine on SnowLeopard and use VNC in full screen mode. No need to upgrade at $$$.
One very minor nit: the keyboard commands to go full screen / windowed on the current beta aren't sticky. But thats very minor.
If you do use CotVNC instead of ScreenSharing don't forget you need to put a VNC password into the ScreenSharing Preference pane on the host machine.
Update: Mikael corrects me that Chicken of the VNC is now more, now superseded by Chicken, and its Chicken that gives me VNC fullscreen capability..