Just a few things to consider. When the famework releases new versions there is always the possibility that existing code wil be broken until you can adjust it for the new freamwork version. Setting up 1 framework for multiple sites can ultimately be a recipe for disaster, unless you take to the time to test everything before implenting the new frame work.

that said, probably the easiest way to accomplish what your after is to put the Zend folder in your php include path. check out the phpinfo(); dump and see where your include path is, then just dump the whole Zend folder in there. You should then be able to include any Zend clases with out having to alter you path in your code.

Again, this can become problematic very quickly if you have multiple clients, multiple applications, etc...