The NYT just quotes Scotty "out the door" McClellan:

Scott McClellan, the White House press secretary, said Mr. Bush's visit to Stanford was interrupted by protesters, who blocked the only road leading to the Hoover Institution, where Mr. Bush was to meet with fellows before dining with Mr. Shultz.

Apparently that was a lie.

Update: Daniel in the comments has links a collected media from the event:

Contrary to what the press is reporting, the road to Hoover Tower was not blocked by protesters. Law enforcement had set up barriers to ensure clear passage long before the protest began, and these barriers were respected by the crowd. However, around 4pm, police in riot gear appeared and attempted to move the crowd from its position on a side-street, East side of Hoover Tower. Strangely, they did not attempt to inform the crowd of the reason why.

Students resisted this move and sat down in the street. That’s when law enforcement pulled a very strange maneuver of questionable legality. They brought in a fire truck with sirens wailing and claimed (falsely) that there was a medical emergency at Hoover. After a lot of verbal abuse from police and firemen, only three protesters remained blocking the truck and these were dragged off (as shown prominently in the photo coverage), arrested, and taken away in a paddy wagon. They’ve since been released on misdemeanor charges. Absurdly, the fire truck then turned around and drove unhurriedly away, sirens off, and the protesters were allowed to fill the street again. Presumably, it had been during this confrontation that Bush’s meeting was re-located.

And people wonder why us kids don't have a lot of confidence in the g-man.