The new formula breaks easier and is chalk-like. If you put it in your mouth it tastes like chalk. Old Armour is a little more difficult to break and has a sweet taste to it.
It's pretty much confirmed now that all scripts that are out there are the new formula. This changed actually started happening in Feb of 2009 and I have not seen any old armour available since May.
ERFA in Canada seems to have the closet old formula available.