I think that Pam told Michael the truth. She is not the lying type. All she really wanted to do was help Michael, who is still really in love with Holly. If it was bad news, I think Pam would have told him so that he could have some closure, and be able to move on. In this situation, it turned out that Holly does still have feelings for Michael. Maybe Michael will be Jim now, as in going after the woman he loves, like Jim did for Pam.
I wanted a showdown between AJ and Michael, it was gearing up towards it, or at least I thought. AJ and Holly would be a cool addition to challenge michael in scranton branch instead of Toby and ryan.. and we will have another love triangle or AJ could be sent to replace michael after hq learns of his miserable lecture tour.. hmm, now that would be a cool episode.
so maybe the writers are keeping that in mind and thats why the letter gives it a chance of happening. It didnt give any hint that the letter is fake so maybe we will be hearing more of the Michael-holly-AJ triangle.
I honestly though that Pam was lying the entire time and I was waiting for her to say something. When Michael was insulting the fat dude in the car on the way back, I thought Pam would get mad and blurt it out but she didn't.