PHPUnit code coverage generation causing memory exhaustion

Do you have xdebug profiling enabled, if so try disabling it. I've experienced this problem before, and it came down to extensions in php (specifically xdebug profiling and/or Inclued heirarchy viewer)

By reducing the number of files included within code coverage, as well as increasing the overall memory limit in PHP, I was able to basically kill this error. The entire Zend Framework was being included within code coverage, which is very large.