I have setup Anyconnect and it currently works from any windows pc. When I try to establish the connection with the anyconnect client already installed on a Mac with Leopard it fails giving a message: "Unable to establish VPN", I believe it is failing on all Max OS X hardware I have around right now.
Can you post the configuration of the ASA? Make sure that you are not using a different port for webvpn or something. Also, what do you see when you run debug webvpn svc 255. You can attach the configuration and the debug as a text file.