Frans noticed the UT Demo-Known Bugs list has moved to a separate page on the Epic site, and has had a single item added to it about the server browser flicker:

Networking
* People have noticed that the server browser now flashes the screenshot and the server title every 5 seconds. This is the server being repinged every 5 seconds. The 3dfx version did this as well but it did not cause flicker. It only does this while browser is visible. It is NOT repinging servers once the browser is no longer visible.