We have found how LEGO do stock control of each pack & box of packs. There IS a unique ID on each packet. It is a series of dot patterns along one edge. A little bit harder than the barcodes, but you can still get the ones you want. See my Useful Links on the right for link to the original Flickr page. Meanwhile, here is what it looks like. See the black dots along the edge below. The gorilla and the samurai are similar, so harder to tell apart in shops:
These charts are for the US packets. We don't know if these apply for the rest of the world yet. There were different barcodes in the US for series 1 & 2.
See earlier blogs (scroll down) to see what the series 3 minifigs look like. Meanwhile I should have some at the market in a week's time if my order from the USA comes in this week.