I guess it is up to personal interpretation but I think Jacob has not imprinted yet. There are several Eclipse references which support this.
1. When Jacob and Edward were talking while Jacob was keeping Bella warm at the campsite. Jacob asked Edward what he would do if Bella decided she wanted Jacob instead of Edward and he responded that he would let her go. Jake said "Just like that?" and Edward's response then was "In the sense that I'd never show her how hard it was for me, yes. But I would keep watch. You see, Jacob, you might leave her someday. Like Sam and Emily, you wouldn't have a choice. I would always be waiting in the wings, hoping for that to happen. " Page 502 Eclipse hardcover.
2. When Bella was talking to Jake the last time after he was injured ( I guess saying goodbye), she said: "I want to love you and make you happy. And I can't, and it's killing me. It's like Sam and Emily, Jake - I never had a choice. I always knew nothing would change. Maybe that's why I was fighting against you so hard." Bella was comparing her love for Edward to what Sam and Emily have. Page 600, Eclipse hardcover.
3. Just before Bella leaves Jakearound page 603, she tells him to be good and do what Carlisle tells him and he will get well faster. Jake says Sure, sure. Then Bella says " I wonder when it will happen, .... When the right girl is going to catch your eye." Implying that Jake will someday imprint on a lucky girl (my interpretation).
I think if Jake had imprinted Bella she would not have been able to resist Jacob, also I think imprinting is spontaneous and not something you can force on the other. It is like embracing your destiny clearly. I think the imprinting process exists between Bella and Edward. (just my personal interpretation though).