Almost embarrassed to admit it, but I got frustrated with no results, then I realized I was typing my search query into the wrong box.

Particularly with the black forum theme/skin, the "search for keywords" box is not completely obvious, and it's not auto-selected as the default text entry point when the page loads, so I was typing into the author box without looking at the label on the left. Doh. Check you're not doing the same.