Oh, one more thing that might be relevant: Fresh install with two add-ons loaded: Firebug 1.2.1 and YSlow. I had Firebug running when testing, but I don't think that is necessary to reproduce the bug.
Firebug doesn't matter, and doctype doesn't matter: the only thing that matters is whether you send the content-type application/xhtml+xml or text/html.
Okay, thanks for the clarification. I noted in the other bug that <script/> elements ARE getting parsed in on a coworker's machine who is running under Ubuntu. Same content, same browser, different environment.