Well, no one appears to be entirely sure, but this is what we know: the Boi was scheduled to go on at 6:20 and was then pushed back to 7 at some point during the afternoon; by 6:50 or so, the area in front of the Sutro stage was completely packed. As 7 and then 7:15 rolled around and the crowd went from anticipatory to angry, it appeared from backstage that Big Boi was definitely there, but that the DJ rig wasn't working, and no one had brought a backup. Dave Chapelle(!) graciously went onstage and placated the crowd with a quick, ad-libbed (and pretty funny) bit. Big Boi's team appeared to have actually gotten everything working in the end — but at that point, we were close to encroaching on Erykah Badu's set on the same stage. So his people came onstage, apologized, told the audience they'd rather not play than give a "half-assed" performance, and officially admitted Big Boi wouldn't be playing (to near-deafening boos from the crowd).
Again, this is all based on what the Express saw and heard from backstage, but we've put in calls to the Outside Lands press people and will update when we hear something official. In the meantime, follow us for real-time observations, analysis, and more.
Update, 1:30: Here's the official word from Chris Vinyard, one of the Outside Lands press people:
Unfortunately, Big Boi had to cancel his performance due to the fact that his DJ had a computer malfunction. They tried to fix it but it got too close to Erykah Badu's set time and Big Boi wanted to respect that. Dave Chappelle was on hand to entertain the audience.
Vinyard also added that "Big Boi and his team were the utmost professional given the technical difficulties at hand, we look forward to working with him in the future."