All About the Proficiency
For the sake of argument, here's the exact text (from the SRD):
Animated: Upon command, an animated shield floats within 2 feet of the wielder, protecting the character as if he or she were using it, but freeing up both the character’s hands. Only one shield can protect a character at a time.
First, it is clear to me that the monk is not wearing the armor when utilizing an
animated shield. That said, I think the relative clause here is, "as if the character were using it". To me that says that the
shield moves and reacts exactly as it would *if* the character had an incorporeal third arm capable of positioning the
shield where appropriate.
Since a monk is not proficient at shield use, I would rule that the
shield would get in the monks way, because she wouldn't know the proper way of making it move, thereby incurring armor check penalties to her attacks and skill checks that involve movement. If the monk spent a feat acquiring shield proficiency, or multi-classed to gain that proficiency, she would know how to properly position the
shield, and so I would then allow her the use of the
animated shield without loss of the character's special monk stuff.
This "incorporeal third arm" (incorporeal so it doesn't interfer with the motion of the body's material components), is also how I would view the
shield in a situation where "attendance" is a factor. Someone trying to sunder or grab the
shield would have to deal with the
shield moving exactly as if the character were wielding it, taking into account awaredness, etc.
And I agree with Caliban on the
ring of force shield, it's essentially deactivated at the beginning of your turn, allowing you to act as if it weren't there, then reactivated at the end of your turn. When it's not your turn, it's there, just like a normal shield. So it's a worthless item for a monk.
Edit: obviously I also agree with Spider, who hadn't posted when I first viewed the page.

Well, almost. I *would* apply the non-proficiency penalty in all cases he mentions.