Comment 3 for bug 545830

As for 0.5.2 (from oneiric) this problem still persists.
Also the comments in squid-deb-proxy.conf are seem to be wrong:

# allow access only to official ubuntu mirrors
# uncomment the third and fouth line to permit any unlisted domain
http_access deny !to_ubuntu_mirrors
#http_access allow !to_ubuntu_mirrors

The third line is uncommented in the default config. If I uncomment the fourth line too, it won't change anything, because squid will deny access to unofficial repository at the third line and will never get to the fourth.
The comments for caching look incorrect too for the same reason:

# don't cache domains not listed in the mirrors file
# uncomment the third and fourth line to cache any unlisted domains
cache deny !to_ubuntu_mirrors
#cache allow !to_ubuntu_mirrors

To allow access to unofficial repositories you should comment the third line and there is no need to uncomment the fourth because both official and unofficial repositories will be allowed by this line:
http_access allow allowed_networks
But according to comment https://bugs.launchpad.net/ubuntu/+source/squid-deb-proxy/+bug/804267/comments/1 this will make your proxy allow access to any site (not just repository) . I configured my browser to use it and it worked.

I'm new to squid so please correct me if I'm wrong.

@Prateek, there is still no client-side configuration required, squid-deb-proxy.conf is a server config.