This is the same problem as a number of other have reported, e.g.
Failed to fetch URL https://dl-ssl.google.com/android/repository/addons_list-1.xml, reason: Connect
In my case, on Linux, the problem was that something had previously created the ~/.android directory with root ownership. So eclipse couldn't write to this directory. Once discovered, this was simply resolved with:
root
sudo chown -R $USER:$USER ~/.android