I plan to do a similar type thing with the Ethernet shield but going at it in the opposite direction.
I will have a dynamic web site (django) that user logs into and can see the current status of the arduino and also send commands. The Ethernet shield will send a POST/GET to the server every second or so (every 10 seconds might be plenty). The shield will send all it's current status on the web request and the server will send back the latest commands from the user.
Still busy getting the hardware stuff working so haven't even plugged in the Ethernet shield yet.