Units of accounting in the unspent transaction outputs in the longest branch of the Satoshi20090103 blockchain (aka "bitcoins") are indeed scarce, as long as Gavin and the miners refrain from changing the protocol; but there is a potentially infinite number of blockchains with the same properties.

If, at some time in the future, a better altcoin comes along (say, with a 0.1 second mean confirmation time, or a distributed trustless way to seize and return stolen bitcoins), and takes over the bitcoin "market", there will be no central bank to swap old bitcoins for new ones at a fixed rate.  Some owners of old bitcoins may be able to sell them to suckers in exchange for new ones, before they become worthless; but many will inevitably lose the money that they invested in BTC.

Bitcoin mining is currently financed by an "inflation tax" of 5-10% per year.  At some point, that inflation will stop and the miners will start to require transaction fees.  No one knows how much those fees will be, but mining is already an oligopoly market so a cartel of 2-3 mining companies can set the fees to whatever gives them maximum net revenue.  By the way, those fees will apply to every transaction, including hotwallet-coldwallet moves, BTC deposits and withdrawals, etc..