Able to connect, but appear not to beable to recieve any packets? Go into the properties of the VPN, go to security and untick “Require data encryption (disconnect if none)". In XP < SP3 this has always been ticked for my tunnels and I've never had a disconnection, but it appears that something in SP3 RC0 makes some behaviour change, but doesn't actually disconnect the tunnel.

I've not discovered why this happens, but this has fixed it on my main desktop and on my virtual machines. I've only had a moment to test this on a PPTP VPN at the moment, but you may find it works for L2TP/ipsec.