The Cisco VPN client 4.7.00 (0510) mostly works for me on OS X 10.4.4, but I get "Error 51" sporadically, and I used to have to reboot to stop it, but not any more.
I took a tip that I found on VersionTracker and did a slight tweak to the command, so that the cvpnd process daemonizes silently to the background and you don't have to remember to keep restarting it:
launchd -d /bin/sh -c \
"while [ 1 ]; do /private/opt/cisco-vpnclient/bin/cvpnd; done"
Hopefully Cisco will solve the problem, so that we don't have to resort to hacky solutions like this.

