Obama and Hasan knew each other pre-inauguration.
"Army Major (Hasan) Played Role in Presidential (Obama) Transition"
I think the delay is likely because Hasan is a Muslim, and the administration doesn't want to look like it's being "too harsh" on Muslims. There is no evidence that Obama and Hasan knew each other, pre- or post-inauguration. The fact that Hasan appeared in the information about some think tank's pre-inauguration conference does not mean that he played a role in Obama's transition, or that he and Obama knew each other pre-inauguration.