In my mind, Google Now is about carving out a space on devices where they can actually show high value contextual advertising outside of search. Their pitch to advertisers will be much stronger if they have a foothold on every device.
I'm also surprised that this was released before Google I/O. Between near confirmation that Android will be 4.3, and the string of major product releases of the last couple of months, it looks like any major announcements will have to come from left field. The Google X people have been dropping hints about some sort of Control Systems thing....