I just run my proxy on port 8080 and set the proxy settings for clients in the GPOs. Additionally firefox needs a pref tweak to make it do NTLM but that's not really a problem.
Ident would be the easiest solution if you wanted to use transparent proxying and authentication. But to my mind it's a non-issue. It's easier to set the clients up with a proxy than it is to deploy Ident servers to all your client machines.