I think services like these are an important reason why gems are so popular amongst Ruby developers, and I figured that PHP really needs something similar. So over the Christmas, I have been brewing on a service, which is now stable enough that I'll make it available to the community at large. pearhub.org provides a place where you can register a project, that is hosted on Github, Google code or similar (Currently only git and subversion is supported). The service will generate a PEAR package and put it on a PEAR channel.
PEAR channels have been difficult to set up in the past but the pearhub.org service makes it simple and you get the added benefit of being able to use the PEAR installer application to do installations and upgrades. You can find out more about the service on their FAQ.