After an idea from bigon on #launchpad today I threw together a tool using the Launchpad API. I've christened this tool ppamadison. It does the same thing as rmadison, but for PPAs. You tell it who's PPA to examine, and what source package to get the information for, and it tells you what versions are available.
$ ppamadison james-w bzr-builddeb bzr-builddeb | 2.0~0ubuntu1~ppa1 | intrepid | source bzr-builddeb | 2.0~ppa1~hardy1 | hardy | source
There's still some things left to do, such as replicating rmadison's odd output formatting, some things missing from the Launchpad API and some interesting things you could add, but the idea is there. One thing missing from the Launchpad APIs as far as I can see is an efficient way to find out which PPAs contain a certain source package name. This would be quite an interesting thing to know.
Would ppamadison be a useful thing to have in ubuntu-dev-tools? If it is worthwhile then I will integrate it. Because this blog post is something that developers might not see, but might be interested in I would then pass it on to the Developer News service, as all it would take would be a quick email, as little as a link to the blog post would do.
(Yes, I am being facetious, but we haven't had a single submission yet)
As an aside, I play with the Launchpad APIs every couple of months and they are getting better, to the point now where most data I want for things I do is available. Thanks to the Launchpad team for their work on it. There are some real problems for some use-cases, such as a cache hit requiring a https connection, but ways can be found to deal with them. In any case, the APIs will allow us to do some really useful things.
P.S. Thank you all for your kind words.