Although my ISP (AT&T DSL) is one of the Participating Providers for ESPN3.com, 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: 184.108.40.206).
forwarded_for off #Needed for ESPN3.com to work correctly
Now go watch the game and be happy.