I found several LEGO stormtroopers on a toy box. They each have a plain yellow head and the words "LEGO 2002" in big letters in their backs. Did they come from a set?
These are promotional items. Not from a set. They can be related to a store-opening, a give-away, other special events, etc. Looking through the Bricklink catalog I found one listing with a stormtrooper torso that you might be referring to: http://www.bricklink.com/catalogItem.asp?P=973pskac01
If you can add a picture we can probably identify them more precisely. Hope this helps some...(c;