what happens in x years when the block chain is no longer measured in mega/gigabytes but in terabytes? at that point every miner in the world holding on to the entire block chain would have to have some rather impressive data storage ability.
Just thinking about what would happen if the writer of this article was successful. http://bitcoinmedia.com/our-next-adoption-phase/
even a small nation adopting Bitcoin for something as benign as paying for cellphone bills would cause an influx of transaction volume the likes of which would make the 10K transaction/day of June look like a stagnant month. The growth of the chain at that pace would be tremendous, and the record keeping nature of the chain (while in itself valuable as a permanent recipt of transaction) would definatly over run my little miner's 40gb HDD.
I'm guessing that I'm missing some truncating function that I haven't seen mentioned... i hope.