Resolution: Not a bug
New output machinery allows content to be compressed before sending over HTTP. This does not seem to work at test.moodle.net servers.
How to reproduce
Open http://test.moodle.net/workshop/theme/yui_combo.php?2.8.0r4/build/assets/skins/sam/skin.css in the browser
Valid CSS should be obtained
Data in a weird binary format. However, if I fetch the same URL via wget, I get proper CSS file. Seems like an issue in content negotiation (maybe wget does not accept gzip or similar).
I consider this as PHP/Apache issue as the new theming works properly on my dev machine.
Please fix ASAP - this blocks any Moodle 2.0 testing at the moment.