Mercurial will have a big impact on how we develop plugins, I think.
Mercurial suits individuals hosting their own packages, and it suits one package per repository. That decentralises development in two ways -- into many packages and many developer sites.
What I'd like to suggest is that we build into the wiki a list of mercurial URLs containing packages, and that the wiki pulls changes from each URL the same way it pulls changes from SVN. Then, we've got a hybrid mechanism.