Don't know if anyone here has run into this problem, but previously VPN (when using Nortel on a Win XP machine) worked fine with AirPort Extreme Base Station (802.11b/g) but for some reason connectivity appears broken on the new 802.11n unit. Here's a thread with numerous users all experiencing the same problem
VPN and AirPort Extreme 802.11N . Anyone come up with a solution yet?