Bill de hÓra is right. I think he's particularly correct that a big part of the reason Git is working well for many projects is the smooth upgrade path/ legacy interaction provided by git-svn. My long bet for the day is that sometime in the near future not providing a DVCS version of an open source project's repository will be seen as a giant middle finger to people who a) want to contribute but b) don't have official "commit privileges."

Note that providing a DVCS repository doesn't mean scrapping Subversion entirely. There currently exist both centralized and distributed versions of the Cosmo repository which, while not totally in sync, allow clients to easily get an up to date git clone of the Subversion repository with minimal hassle. Instructions for that can be found here.

I spun my wheels for a while trying to figure out the best way to set up an "official" git-svn clone of the Cosmo repository before realizing that most of the complexity (there isn't much) is on the client side. For the record, however, thus far the process has been:

mkdir cosmo
cd cosmo
git svn init http://svn.osafoundation.org/server/cosmo -s
cd ../
mv cosmo/.git cosmo.git
rmdir -rf cosmo
cd cosmo.git
git --bare svn fetch # this will take a while, go get a beer
git --bare update-server-info
chmod a+x hooks/post-update

In the future I'd like to add some smarts to periodically pull down the latest commits from the Subversion repo, but since clients can do this cheaply and independently it's not actually a big deal.