I have a hardware vpn now, and when I was using the built in VPN I was also using Tiger, so I appologize if this information is outdated.
When I was using the built in VPN client I was able to resolve the iChat connection issue by doing two things. The first was opening up the VPN settings and unchecking the "Send all traffic over VPN connection". The second was setting the order of the connections so that the VPN connection was lower in the list than airport and ethernet. On Tiger the second was found in the Internet Connect application, and I believe it's been moved to the Network settings, Set Service Order option under the action menu (the gear icon) at the bottom of the list of connections.

Seconded. I've been using Shimo for awhile now and agree - it's much easier to use and solves the occasional problem I had with the actual Cisco client.

What is Shimo? I've never heard of it.

I find the sudo command also stops a nagging problem where the CISCO VPN simply doesn't connect, and finally comes back with the message 'closed by local client' or something to that effect.

In one case, I had to use the sudo command twice to fix this.

Thanks, Rob. This is really helpful!

For a few of my Macs, disabling the Firewire port from Networking has solved the error 51 issue. (Mac OS X 10.5 with the Cisco VPN Client 4.9.01 (0100). The only side effect is this will disable ad-hoc networking using Firewire, something which I have never used anyways.

System Preferences > Networking, select the FireWire port and make it inactive by clicking on the gear. Then I click the minus button to remove it from the list.

This was based on someone else's discovery and posting on another site.

I would like to know if these resolves the issues for other users as well.

I support our VPN in the enterprise and am very familiar with that error. You can update your tip to include a double clickable applet made with apple script. All you need to do is put the below line in script editor and save it as non editable then distribute to users. When run it just asks for admin auth. Sometimes I find even running this doesn't fix the VPN and in those few cases only a reboot will cure it. So far its worked well enough to keep around. Cheers!

do shell script "SystemStarter restart CiscoVPN" with administrator privileges

Cisco has also posted a fix:

http://www.cisco.com/en/US/products/sw/secursw/ps2308/products_qanda_item09186a00801c2dbe.shtml

This fix did not work for me, and neither did the Macworld fix, but my Terminal message suggests I am missing a bundle. I will try to re-install the bundle by installing / re-installing the most recent client tomorrow.

The Terminal message I get is:

sudo kextload /System/Library/Extensions/CiscoVPN/CiscoVPN
kextload: /System/Library/Extensions/CiscoVPN/CiscoVPN: no such bundle file exists
can't add kernel extension /System/Library/Extensions/CiscoVPN/CiscoVPN (file access/permissions) (run kextload on this kext with -t for diagnostic output)

Am I guessing correctly that an install of the most recent client should correct this? Or is there also a permissions problem here? I am clearly not a Terminal pro.

Thanks in advance for your thoughts.