I actually spent the better part of today setting up subversion repositories for the bulk of our projects at work.

The solution we finally settled on was using svn:externals pointing to the release tags. Each site we produce now has its own repository. Each repository has a svn:external property pointing to a release tag in the zend repository. When it comes time to update the framework, we simply change the svn:externals property to point to the new release tag and update, test, repair, test, check in, then the live site gets an update as its just a working copy of that site last stable release tag.

The bad side of this is that everysite has a copy of the zf code. Space isn't a problem for us at work, but at home or on a more limiting hard drive you might still want to do the symlink thing.