In season Two, Don pays the elevator attendant to deny access to the elevator when Sterling and Draper show up later. Why would Draper do that considering that both Draper and Sterling would have to climb stairs all the way to the 21st Floor? Was he trying to teach Sterling a lesson of some sort for hitting on Betty the previous night?
That's exactly it ... Don was getting revenge on Roger, upset that Roger had made a pass at his wife. Hence Don pushing the martinis, oysters, and cheesecake on Roger, knowing that it would be a lethal combination which, when combined with the long stair climb (which Don, being quite a bit younger, knew he could handle), would lead to the projectile vomiting in front of the clients.
The season six premiere called back to that episode when Don vomited at the funeral of Roger's mother.
(Also note that the episode you reference was not in season two, but in season one --- episode seven, entitled "Red in the Face." Here's the scene in question on YouTube: http://www.youtube.com/watch?v=gzsJsbGLTUY).