Since upgrading to the latest version of Remote Desktop (v 3.2.2), I've noticed that many of the computers in the client list are not 'available.' Their status is listed only as 'Screen sharing available,' and that's all I can do ... all of the added functionality of Remote Desktop is no longer available.
In our research lab, we keep all of our client computers with the same setup, but somewhat frustratingly, only some of the clients were exhibiting this problem. Inspection of the Sharing setup from System Preferences revealed no difference between any of the clients.
I then checked the firewall settings on each client. We run the default OS X firewall with the "Set access for specific services and applications" setting. It was then that I noticed that the ARDAgent application was appearing in the list of specific applications, but it was set to 'block incoming connections' (only on the problem computers). Changing this back to 'Allow incoming connections' fixed the problem. However, one client did not even have ARDAgent in the list, so I had to add it manually (the application lives in /System » Library » CoreServices » RemoteManagement), and that worked as well.
I don't know how or why the firewall settings were changed, but I'm happy now that I have a way of fixing it.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20081007134846705