UPDATE: The Scene reports that the undercover van belongs to Metro Police, who decided for some odd reason that they needed to provide Donald Rumsfeld extra special coverage when he was speaking in Nashville.
Today, Metro Police spokeswoman Kristin Mumford told the Scene that it was actually undercover Nashville cops:
The van was a Metro Police Department undercover vehicle that was part of the dignitary protection plan for Donald Rumsfeld. Given his obviously controversial status, and the police department hearing talk of possible disruptions to his visit, officers were assigned to proactively work to ensure everyone's safety.
Because everyone knows Mr. Rumsfeld cannot afford to pay for private security himself or avail upon his buddies whom he hired to go to Iraq as private security contractors.