Apple Remote Desktop 3.5 Admin
Apple has released Remote Desktop 3.5 Admin, an update to its remote desktop management software that introduces several reliability, usability, and compatibility improvements — at least some likely connected to the company’s recent release of Mac OS X 10.7 Lion and Mac OS X 10.7 Lion Server. An updated version of the app’s companion Dashboard widget has also been released. ($79.99 new from the Mac App Store, free update through Software Update or the Mac App Store, 25.27 MB)
This update logged me out of screen sharing with 10.6 server and I'm unable to log in again.
It may be that you need to install a client update on the server as well, though I didn't see anything about that in the release notes. I don't have trouble controlling a Mac running 10.5 Leopard or 10.7 Lion from 10.6 Snow Leopard, but I don't have a 10.6 Snow Leopard Server machine to test on.
If you need to run Software Update on the Snow Leopard Server machine, you can ssh in and run it from the command line. Fussy, I know, but a necessary fallback sometimes when ARD doesn't work.