Pathfinder had already become #1.
Now, for a short time, it will be the undisputed king because the wind will be out of 4E's sails and "iteration" is just a working theory.
But when "iteration" hit shelves it will be HUGE for some period of time.
Last time around the brains over at Paizo NAILED their tea-leaf readings. We will see if they can do it again.
In the short term they have a window to gain some fan base.
WotC has a very tall order trying to rebuild D&D. Failure is certainly possible. If that happens and Paizo holds steady, they will pretty be the new name in gaming for a generation. (or at least, they will have that title as theirs to try to not screw-up....

)
If WotC does "OK", then Paizo will still be in good shape in a 1 - 2 relationship with either potentially being distinctly ahead or it being too close to tell.
If WotC hits a home run then Paizo will likely need to seriously react.
But in any case this is going to be a slow moving train. But it still won't turn easy. A lot of people saw where 4E was headed before it was even officially released. And Paizo played that well. They have a vastly stronger driver's seat position now, so I think they can react to anything, be it Open or Closed, Awesome or Tailspin.
I do bet their development strategy just got a bit more risk averse however. And I bet they announce things a bit more slowly and deliberately as the lay of the land is revealed.