The BBC reported it as

"Comedian Robin Williams performed an impromptu stand-up routine before the delayed recording of a BBC World debate on the future of news."

The TED blog added more detail:

During the BBC World Debate hosted at TED2008 last month, a brief technical delay threatened to become an awkward, show-stopping break. Then a heckler stood up in the crowd. As Wired’s Epicenter reports:

… a voice behind me spoke up, presumably a heckler, and began speaking loudly as if he were conducting a live news feed, joking that he was reporting live from TED but … was "wondering why at a technology conference everything is running so shittily" (at least that’s the word I think he used; it was hard to hear the last word through the audience’s laughter).

It was Robin Williams, and he ran onstage to do a 10-minute improv in front of Queen Noor, Sergey Brin and the rest of the BBC panelists. Video or it didn’t happen, you say? Here’s Robin Williams’ improv, or 2+ minutes of it, from the BBC website.