we run all our internal sites off the same server using vhost directives in apache

by putting the library in the php include path it will be available to all sites hosted by that server

We recently moved away from this solution however. What we do now is manage the Zned_Framework version through subversion. It works like so:

Each site has a subversion repository (svn://uri/siteA).
Since our sites all use a modular conventional FS layout (which includes a library folder parallel to the document root) we can easily attach a svn:externals property to the library folder and point it to the release tag of choice in the Zend repository.
When we need to update the version of ZF the site is using, we just update the svn:externals property and update to a development server where we can test and fix any issues. Once everything tests good we can update the live server via a svn update or switch to a new release tag for our site.