You can add me to the list of people getting kernel panics while running the VPN actually connected. Since the 10.5.3 update (including 10.5.4) it's gotten much worse. Starting with 10.5.3, I can usually get a panic within 24 hours of bringing the VPN up and transferring data across it. If it just sits idle, not transferring data, then it doesn't panic. We have a split VPN, so only traffic bound for our corporate network goes over the VPN, not regular internet traffic (which alleviates the problem *slightly*).
I'm getting this one two separate systems. One is a Quad G5 and one is an MBP. I've also been in contact with at least two other people also having this problem. At this point I'm convinced It's definitely the Cisco VPN. And yes for the most part it should be up to Cisco to fix, but there are things Apple can do to help as it's in Apple's best interest to not have it's OS crashing. Apple has a working relationship with Cisco (don't forget apple implemented the Cisco VPN client themselves on the iPhone) and can work with them to fix the problem.
I've filed a bug report on this to Apple (ID#6080441). No response from Apple yet on it.
Would be nice to track here how many people are running into the problem, might be able to put more pressure on Cisco/Apple to fix the problem.