You do have to be a U.S. citizen to vote.[1]

[1]http://www.usa.gov/Citizen/Topics/Voting/Register.shtml

Not only that, but accidentally voting (e.g. because the person at the DMV said you could) can bar you from ever becoming a citizen and even get you deported:

http://www.immihelp.com/forum/showthread.php/91741-Denied-N-...

http://www.hg.org/article.asp?id=27445

http://www.gmanetwork.com/news/story/141110/pinoyabroad/pina...

https://beki70.wordpress.com/2011/03/03/fortunately-i-wasnt-...

Some states and local governments allow voting by non-citizen residents. They still can't vote in federal elections, though.

It seems like green card holders, who in some cities can vote in local elections, can not vote in SF.

'Airplane!'? https://www.youtube.com/watch?v=SmHeP9Sve48