I really love the concept and technology here but I am still trying to wrestle with the implications. I see two potential problems if bitcoin was to become a very popular currency/system.

• The number of blocks is infinite and the history; the complete chain is important. Even though blocks are small (at least today) I am concerned that this system relies on each peer having a complete record of the entire chain. After many years and supposing hundreds of millions of people generating blocks, wouldn't the size of the chain become untenable?
• Closely related to the above point: As the number of transactions per-second go up the size of the blocks also go up. If the system ever got to the point of having hundreds of thousands of transactions per-second the model seams unsustainable. One could argue that the number of blocks generated would also increase but this only exacerbates the the issue I pointed out above.