Ruby has Gemcutter, Python has Pypi, and now PHP has Pearhub!
img http://img.skitch.com/20100108-j2gtgapb9a1s11fqeukw6tqfdt.png http://pearhub.org
Whether your project is hosted on GitHub or somewhere else, Pearhub makes it easy to publish. Check out the FAQ or Introductory Blog Post for more info.
(Oh yeah, best part: Pearhub encourages (and works best with) Semantic Versioning)