For the amount of web developer job interviews I've been to, I'm somewhat surprised that no one has brought up Zend certification. Has anyone ever been certified before, or is it a waste of time/money?
As a full time PHP Developer, I have never had anybody ask if I was Zend certified.
I have not taken the certification exam, and I don't foresee myself taking it in the future. While it may give you one up on your competition in an interview, I think the same thing can be achieved by simply knowing your craft.
By all means, take the exam if you like, but I don't think it will help you in your PHP career.
Also, when I DID look at the Zend certification (back in the early PHP5 days), I noticed there was a lot of weight put on memorization of function names and parameters. I don't feel this makes you a better programmer, which had a huge weight in my decision not to take the exam.
I tend to shy away from technology-specific certifications for the reason that Craige mentioned in his answer - they focus on things like functions, parameters, and language features that change (sometimes rapidly) over time. I would rather see projects and work history using technology, rather than certifications. These would demonstrate your grasp of language functions and show me how you design and implement systems. The certifications that I would look for are those by well-respected organizations that offer technology-independent validation of knowledge about a given set of concepts.