The Cisco implementation VPN client on the phone doesn't work with GNATBox right now. Being hard core IPSec guys we didn't muck around with PPTP - that might work.
I did hear a rumour someone high up at GTA might just have an iPhone and it's being closely looked at - ;)
I didn't see anything obvious for setting up a VPN over PPTP in the GB documentation? Is that a newish feature? I only recently starting the move from 4.0.4 to 5.1.3 (GB-Ware).