At Jezebel, Tracie Egan Morissey has a similar take to to mine (in my Mad Men season six premiere review):
If death was a lingering threat lurking in the shadows throughout all of last season, it’s now taken the spotlight. Of course this season would be all about death and we were foolish to think that once it happened to Lane last season, that the show would go on to another topic. Death doesn’t go away after it happens. It just continues to happen.
And it seems as though this season might be trying to explore what happens after death. We get our first clue when we see Don’s idea of some beach reading: Dante’s Inferno. Hell is the first of three the realms of the afterlife (followed by purgatory and paradise) featured in the Divine Comedy. So there you have it. Don is in hell, and according to his broken watch, time has stopped.