I am sensitive to this and never use "guys" when sending an email. So you can imagine my surprise a few years back when I overheard a female teenage soccer player address her female teammates during a scrimmage with "hey guys...".

edit: just asked my 14 y/o (going on 30) daughter. She regularly uses "guys" to address her girl friends and went so far as saying "what am I supposed to call them? Gals? Girls? That sounds wierd."

edit 2: wife states she'd use "guys" to address a group of women.

Background: raised in Miami by northern parents, currently residing in NC. Wife raised all over (airforce brat). I'm not immune to using "y'all," but "all y'all" is a bridge too far.