If you have a large number of players, you probably want the DM to track initiative just to maintain a semblance of order. In our Friday games, the DM uses index cards to track initiative and calls out each character's name when it is his or her turn. On the rare occasions when somebody isn't paying attention, they get a nudge. In the more likely event that they left the room for a bathroom or smoking break, the DM usually rules that they delay until they get back.
For my games, I use index cards with player info on them. That way, I can just tell a player if they are getting it and by what. Speeds up the game and makes for a better game.
But, if a player decides to get up and leave, they lose their turn that round. Bathroom breaks are ok. I'm really interested in seeing how to handle combat with HMB. I'll know next Tuesday when I have the book.
I'm super excited for it.