Good news - the Zend Framework License is now compatible with the GPL. This means you can use the ZF for your own GPL-licensed PHP project.
From Andi's email:
We have decided to change the license of the Zend Framework from a PHP-like license, to the simpler new BSD license. This was after many discussions we have had with the community and the partners.
The big change is the removal of the advertising clause:
Redistributions of any form whatsoever must retain the following acknowledgment: "This product includes the Zend Framework, freely available at http://www.zend.com/"
Note: Personally, I don't like calling this an advertising clause, because advertising isn't necessarily the primary purpose. The reason the PHP license includes such a clause is to make sure people don't get fooled into thinking they must pay for something which is available for free.
In addition to compatibility with the GPL, this change also has the added benefit of using an existing license, so it's not necessary for companies to evaluate yet another license.
Compatibility with the GPL isn't something I appreciated in the beginning. There were a few complaints about the license not being compatibile with the GPL (some that seemed to suggest the GPL itself as a better license), but it wasn't until I discussed this with Zak in Montreal that I learned the root cause of the incompatibility. Although I think the advertising clause has value, I think Zend has made the best decision in this case.