I'm too cheap to buy a stamp and see what it looks like but I imagine it will be one of those groovy 2-D barcodes. In principle, this is a great idea but I just can't help thinking they've probably slipped up somewhere.
I assume there are already cryptanalysts looking at this. My guess is that it will be cracked within two weeks and we will see a "Royal Mail suspends online postage 'experiment'" story shortly afterwards (it will be rebranded as an experiment after the fact).
Any advance on two weeks?