Git core has support for CVS, SVN, Perforce, Arch and Quilt.

There are plugins for Mercurial, Bazaar and DARCS.

https://git.wiki.kernel.org/index.php/Interfaces,_frontends,...

Git to Mercurial.

http://hg-git.github.com/

Git to Bazaar.

http://doc.bazaar.canonical.com/migration/en/foreign/bzr-on-...

It doesn't seem other DVCS are that incompatible.