As far as I'm concerned the only 'Guest Mode' I need on my phone is the emergency call screen. I'm totally willing to be the 'weirdo' who won't let someone use his phone.
I was thinking of something related to this earlier today. One thing I'd really like from the emergency call screen is to allow me to tag several of my contacts as emergency contacts. That way if my phone is stolen, or I am hurt in an accident, the list of proper people to notify is very apparent.