I am experiencing the same problem. The problem resides not in the ADT repository but in the Eclipse (Galileo) repository.
There are certain dependencies for ADT that need to be installed in order to install ADT. At this moment the repository for Eclipse doesn't seem to work. It can't install the dependencies.
So far i have been able to locate some of the dependencies, downloaded them by hand used unpack200 to unpack them and put them into my ~/.eclipse/org.eclipse.platform_3.5.0_155965261/plugins but i guess they have to be registered somewhere else because i'm stil getting errors on the same file which is actually existing in the plugins directory.
The weird thing is that when i browse to the repository source using Firefox the site does open up. Yet if Eclipse tries to download the required filed it fails, which is not an Eclipse problem because wget or firefox are also unable to get the files.
I've been trying to get it to work for several days now, because i thought that it might be able for download.eclipse.org to be down for a few hours perhaps. Yet it's still not working.
Here's part of the error message i'm getting:
An error occurred while collecting items to be installed
session context was:(profile=PlatformProfile, phase=org.eclipse.equinox.internal.provisional.p2.engine.phases.Collect, operand=, action=).
Unable to read repository at http:/ /download.eclipse.org/modeling/emf/updates/plugins/org.eclipse.emf.common_2.5.0.v200906151043.jar.pack.gz.
The server download.eclipse.org failed to respond
Unable to read repository at http:/ /download.eclipse.org/modeling/emf/updates/plugins/org.eclipse.emf.ecore_2.5.0.v200906151043.jar.pack.gz.
The server download.eclipse.org failed to respond
and that list goes on for a couple of more files...