There is now more evidence that Plan B does not prevent the implantation of a fertilized egg in the uterus. This makes perfect sense, being that this is how birth control generally works as well, and Plan B is just a bigger dose of normal oral contraceptives. I won't be satisfied that it never prevents implantation until there is more rigorous testing, but that may never even be possible. Either way, it's pretty clear that in the majority of cases, EC prevents ovulation rather than implantation. And I should point out that the effective use of emergency contraception prevents abortion 100% of the time.
By the way, if you haven't happened upon it, the place to go for the best picture of current research on any life science issue is Pubmed, which keeps a searchable index of scientific papers, lots of abstracts, and even some full-text articles. If you've got more questions, take them up with the papers there.
Link via Feministe.