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.

Reversing the message he gave two weeks ago before MacWorld that his dramatic weight loss was a matter of a “hormonal problem,” Steve Jobs today announced he was taking medical leave from his job as Apple CEO, saying “my health-related issues are more complex than I originally thought.”

Trading in Apple shares was temporarily halted, with the shares losing 8% when after-hours trading resumed.

According to a post on ZDNet’s Apple Core blog, so many people attending the MacWorld conference in San Francisco this week are using the highly touted cloud data-driven features of Apple’s iPhone that it’s almost impossible to access AT&T’s 3G network.

Phone calls are hit or miss… About half of my calls don’t connect, fail in the middle or otherwise have weird silent dropouts throughout the call… Data connections on AT&T are brutally slow… I thought SF was supposed to be wired?

Maybe it’s just as well Apple is dropping out of the show next year. With attendance this year already down, it should really fall off when the main vendor doesn’t show up — if the conference happens at all. So maybe next January there’ll be no problem at all making pizza orders on your iPhone.

An Apple Worldwide Developers Conference (WWDC) starts at Moscone today, and thousands of feverish geeks are gathering in anticipation of the usual announcements of new products. Expectations for a next-gen iPhone were so high that Twitter suggested a search string so breathless fans worldwide could vulture the news. But if you’d like your updates in English sentences, SFGate is liveblogging Steve Jobs’ announcement. Also liveblogging is Gizmodo.