I've this annoying problem.
I've a pix525 with 8.0(3) release, and I've enable Nat-t, with
enable ipsec over nat-t
checkbox (from Configuration - Remote Access VPN - Advanced - IKE Parameters).
Pix525 is connected in failover configuration to another pix525.
When I reload one firewall, this configuration disappear, so I must reenalble it everytime this happen.
I've noticed that this happen also with a couple of ASA5510 and of PIX515, with same release.
If I disable failover (and I use only one firewall), this doesn't happen.