Hi Kim - I have this boiled down to SOMETHING (like the Ultimate SEO module) not being compatible with either PHP 5.X, or the latest Zend Optimizer.

As a workaround, I have recompiled our Apache and PHP back to 4.X and now the site is back online using our original SEO and files.

It's a little difficult trying to troubleshoot the issue when the only way to reproduce it is to effectively upgrade my server and knock my site offline (since I can't simply install a "test" store to troubleshoot unless I have another server).

So in the meantime, I am going to leave things as is since they appear to be working with our old PHP and Apache.

Thanks for the suggestions.