KavazovAngel: I just tested something... Apparently, you only need a guest pass from a friend to unlock the single player portion of the game, meaning, the single player campaign and the challenge missions
ARE ALL FREE. I think Blizzard kind of put themselves in a stupid situation here. :)
If you don't play online, you don't even have to buy the game to play the campaign.
Hm. The more I think about it, the more likely it sounds.
The guest passes state that they are good for a total of 7 hours of playtime, or 14 days duration, whichever comes first.
But if you have no internet connection at all there is no legitimate way to track that. Saving it locally wouldn't work as it'd just get sniffed out and cracked.