I know I'm a bit late on this topic, but came across it while looking to implement the same type of thing.
Regarding the dynamic IP with the wifi shield, my plan is to just have the client (arduino project) connect to the server (which has a static ip on local net) and pass it's ip to the server. That way it doesn't matter whether it's static or dynamic. The server will always immediately know what the client's IP is.
Just thought I'd share hoping it would help someone.