Although my ISP (AT&T DSL) is one of the Participating Providers for, I was having trouble getting it to work correctly after setting up a Squid based transparent proxy server on my home network.  It turns out there is a Squid setting you have to tweak to prevent ESPN3 from blocking you.

Make sure your /etc/squid.squid.conf file has the forwarded_for setting turned off.  By default it is on.  This setting determines if you local client system IP address is included in the HTTP requests it forwards (i.e. X-Forwarded-For:

forwarded_for off    #Needed for to work correctly

Now go watch the game and be happy.