Zend Optimizer works perfectly with EA in my production system under php 5.0.4 so you have no trouble in your setup.
eAccelerator is a updated port of TurckMMC and even back in TurckMMC days, the "code optimizer" was very buggy. It is, I believe, still very buggy at best. One thing you learn about adopting software is to use only the best part of one piece and combine the best pieces. The goal of eacc product is to cache code, not to optimize, so optimizing is not high on their agenda thus less people testing it which means to more bugs.
Stick with Zend Optimizer for code optimization.