(please excuse the note form of this article)
Background
I am involved in a number of drupal projects, both new and legacy, some as paid projects, some volunteer.
I am involved or want to help with the maintainence of the code base (security patching, upgrades etc).  However not all of the projects are on my own server.  And as I am not a hosting company that is how things should be.
However, patching and upgrading is a pain, especially over many sites.  How could this be managed efficiently and cooperatively?
The Solution in general