In the last few weeks I've both passed the certificate myself and also taught Zend's certification training course as a classroom course at NTI Leeds. I thought I'd share my top tips for preparing for taking the ZCE - getting to the standard, last-minute preparations, and also some tips for surviving the day itself (disclaimer: everyone sitting the ZCE signs a declaration not to disclose the contents of the exam, so I can't actually tell you the questions, sorry!)
She recommends that only developers with at least a year or two of using PHP regularly take the exam due to the amount of PHP knowledge needed. If you haven't at least touched on most aspects of the language, you might have a difficult time with it. She also points out the lack of up-to-date training materials, some of the things she did the night before as a refresher and a few tips for specific parts of the exam.