Hi,
I had exactly the same problem, i couldn't get L2TP/IPSEC working on a 3600 but i could on a PIX.
I got as far as getting L2TP working on the router but struggled with IPSEC. I've configured IPSEC lots of times between routers and also for remote access with VPN client but couldn't get L2TP/IPSEC working on a router.
Also recently discovered that PIX software version 7.0 has dropped support for L2TP/IPSEC to allow the addition of new features.
Rgds
Paddy