How about linking to the project web page or the official blog where
the move was announced when available? I think that's how it's done on
the mercurial page. And it explains people why the switch was done
rather then linking to a source repository they might not care about
and the link to the project page might give a hint about the
importance of the given project for those that might not know it (such
as prototype, mootools or liftweb).

example:
http://weblog.rubyonrails.org/2008/4/2/rails-is-moving-from-svn-to-git