In the original games you could pause single player. In Diablo III, you are always connected and there is no real "single player" mode. Is it still possible to pause the game? I haven't been game (no pun intended) to test it out in anger.
If you press Escape while no other player is connected, the game pauses, just like many other single-player games.
You can definitely press Escape for pause but keep in mind that game will kick you in about 10 minutes of "Pause" with timeout error. That's not a real pause!