VPN with Certificates

I am looking into utilising certificates for secure VPN connections. I
intend to use an internal Microsoft CA installed on windows server 2003

So far as per the MS documentation I have installed Certificate Services and
have installed a computer certificate on a test VPN server. I have also
changed the VPN server policy to only allow authentication method to
certificate only.

On a test laptop I have connected to the CA and requested a user certificate
for a test user and have configured the VPN client connection to use this
certificate. (Windows XP Client)

When the VPN connection on the client is clicked this connects directly to
the test VPN server directly with no username/password prompt. (This is a
non domain laptop)


Issue:

If I revoke the user certificate in Certificate services on the server for
the test user, this user can still connect to the VPN server with no
challenge! Why is this?

What am I missing?