The British company acrossair is supposed to expand its “Nearest Tube” application for the iPhone 3GS — changed to “Nearest Subway” for the US market — to San Francisco today. The app uses GPS to overlay a subway map on the phone’s live video feed to tell users where the nearest station is.

It should be a simple feat in San Francisco, which has only two subway lines. But checking the acrossair website shows information only for the London and New York versions, and even those are said to be “launching as soon as Apple approves it!”

Update a day later: Speaking of BART, Streetsblog draws attention to the transit system’s “data transparency,” meaning that it allows anyone to use live data from its train control system to build applications, and lists the resulting applications on its website.