I found some discussions about Linksys/VPN/Cisco router here and just want to report back my case.
I have a Linksys WRV54G at home connecting to the Internet via aDSL with dynamic IP address. It connects just fine using IPSec VPN to the Cisco 2811/AdvanceIP/K9 in my office.
I did upgrade the firmware, but I made that upgrade many weeks before I tried the first VPN connection to my Cisco router so I am not sure if the upgrade is a requirement for the successful connection.
DT