Step 1. Start a game of UT3 - check the 'Record demo' option.
Step 2. Rock.
Step 3. Click 'Community' on the main menu and double-click your demo.
Step 4. Press Tab (!) to bring up the console, and type 'slomo 0.2' then 'showhud 0'.
Step 5. Observe self rocking.
This post has been reduced from its original form since my machine crashed and lost the entire freaking thing.