So Ben and I have both had this problem, the VPN client wouldn't launch no matter what with Windows 8. Turns out all you have to do is edit one registry key and it'll work again!
You might see error messages like “The VPN Client driver has encountered an error.” or “Cannot initiate VPN.”
...
Browse to HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\vpnva
Double-click on the DisplayName key, and remove the INF information from the display name so that it appears as Cisco AnyConnect VPN Virtual Miniport Adapter for Windows x64. (or the 32-bit / x86 equivalent)
Thanks to "Tom on Tech"!
via http://www.tomontech.com/2012/03/pro-tip-cisco-anyconnect-vpn-client-and-windows-8-consumer-preview/