Eclipse -> Preferences... | General -> Network Connections
You have to change the drop-down the Manual before you can edit the HTTP and HTTPS proxies. Since I have adb mapping 8080 for the proxy, that's what I am using in this proxy. Once the proxy is set in Network Connections, plugins installed through regular means.
Network Connections for All
I had to install the 1.6 platform so I could create a new app for my phone. Since the Android plugin manages the platform and API installations, I wasn't real confident that the Network Connections proxy would get used. I was pleased to find that Network Connections does affect the ADT properly - I'm somewhere in Iowa between Des Moines and Council Bluffs and getting the platform.