Google+ comes close, in that there is a "Share" button at the top of the page, but it disappears when you scroll down, and doesn't reappear again until you get to the top, WHERE THE MAIN STATUS-UPDATE FIELD ALREADY IS. This is just dumb.
Just put a little "Write" button next to the search box. Get rid of the main "Share What's New" box at the top of the feed. Make it simpler.