I had a boss who despised knocked out type -- white letters on a black or colored background. The argument, which I never saw definitive proof for one way or another, was that it's easier to read dark text on a light background -- books being a prime example. (Something Mad Man David Ogilvy pronounced from on high 800 years ago.)
Black on white may be more effective for communications, but so are capturing attention and variety. It may be why Helvetica, like Hansel, is so hot right now, until it becomes so pervasive that something else starts gaining traction. (Heidi Klum: Helvetica, you are out.) After a while the font du jour will also begin boring people who will return to "classic" Helvetica. See also: skinny vs wide ties and lapels, long hair vs short, black vs color, Democrats vs Republicans, and Charlie Sheen.
But back to knocked out type. I first noticed this apparent trend on the Bloomberg iPhone app -- but interestingly not its website. Now it's popping up everywhere. I'm not sure what the movitation was for using white type on a black background -- I suspect someone at Bloomberg thought it looked sharp or high-tech or maybe simply the opposite of Apple. Nevertheless, I found I've had no trouble reading white longform copy on my phone. I haven't gotten a migraine or started drinking martinis for breakfast or smoking a pipe. Maybe David Ogilvy was full of crap -- or maybe he'll be in again tomorrow.