In summation and clarifying some confusion, here's what I've pieced together:
1) Diablo 3 demo = starter edition = guest pass (ie, only one non-payed option available)
2) Presumably to reduce initial server load, guest pass code is needed to play the demo for the first 30 days
3) After 30 days becomes "starter edition" available to anyone
4) Guest pass is only available via boxed game, not online purchase
5) It's the same downloaded client software for everyone, even though you're only playing one act
Number 4 is the bummer for me, I wanted to try it out on my laptop to see if it was playable enough, but all my friends got the digital download. I'll roll the dice after the weekend if I can't snag a guest pass before then.