I know of several people who had already heard from Penn. However, I hear that Penn sends out interview invitations in a couple of rounds.
For your questions, the whole rejection letter vs. acceptance letter vary depending on school. UWashington sent out some rejection letters before invites to interview, but UPenn sends out rejection letters after invites to interview, I think
Interview requests come out, I believe, mostly in late Jan to mid Feb. But the West coast schools seem to do it earlier...
This is just what I think, which could be wrong.