The http://packages.ubuntu.com/ site provides great information on packages, with changelogs and much more, but only covers currently supported releases.
The rmadison program (devscripts package) will also give me command-line access to all versions of a package from currently supported releases (nice!):
$ rmadison gnome-mount gnome-mount | 0.8~svn20080225-0ubuntu4 | hardy | source, amd64, i386 gnome-mount | 0.8-2ubuntu1 | karmic/universe | source, amd64, i386
But I don't see a way to find e.g. what version was in intrepid, which is no longer supported. I know the intrepid release is available at http://old-releases.ubuntu.com/ but I don't want to have to go through each repository by hand.