Are we required to avert our eyes or expected to admire?

Why not simply appreciate the view without drooling? If she's in public view, she obviously intended to reveal her assets. If they are pleasing to the eye, then a short glance is acceptable. Starring at them with your tounge hanging out and your eyes bulging is tasteless.

Girls check out guys who strip off their shirts to show off their six packs or 12-packs or cases, so why shouldn't guys check out clevage that is open for view?

They're just breasts. Every woman has them, and some men have them.

With that being said, don't stare, don't smile a creepy smile, but if your eyes happen to wander now and then, well then that's not really a problem, is it? And if anybody gives you some crap about, "Hey! She's/I'm up there/here!" just shrug and reply with this old standby:

http://www.youtube.com/watch?v=WAENcj7NKS8

From about 5:00 to 5:30

It's okay to notice, but not to stare. You should never do something that might make the woman uncomfortable.

If I don't look away I start drooling and making motorboat noises.

If they don't want you to look, they should not show them. Most girls want to look. European and English girls like it , but be careful of american girls because they get aggressive if you look too long.

It may be visible to everyone, but not meant to lure EVERYONE.

Ironic.

I usually default to turning my head since I KNOW she is not after me.