One possible answer to central mint vulnerability is bit gold -- a currency the value of which does not depend on any particular trusted third party. Another alternative is an object barter economy.
The key ideas of this nanobarter scheme are
(1) the stuff to be traded (in Tahoe, disk space for backup) is represented by digital barter certificates (same protocol as digital cash, but every node is a "mint" that issues its own barter notes), and
(2) default barter order and an agent ("market translator") that translates user behavior into barter orders. In the disk space economy, the default barter order might be to simply do a periodic barter that backs up N gigabytes of other peoples' disks in exchange for N gigabytes of their own. Many more sophisticated barter orders are also possible.
If the reader is familiar with Menger's account of the origin of money from barter, this scheme is quite in the spirit of his scenario -- except that we reduce the transaction costs of barter by brute force automation instead of by making everybody choose a single currency.
The transaction log and accounts are presented to the user in terms of a "pet currency"; the market translator automatically converts all different kinds of barter note prices into their pet currency values whenever prices need to be presented to the user.
Every computer on the network (called a "node") runs a "mint" that issues "currency" (barter notes) backed by its commodity (e.g. disk space). In a simple system all disk space barter notes are treated as equivalent. Or there might be L different currencies corresponding the the L different kinds of leases in Tahoe. (In Tahoe a certain amount of disk space on a foreign disk is "leased" for a certain period of time). Indeed, a barter note is simply a lease in bearer form -- it can mean "I promise to bearer to provide G gigabytes of disk space for D days", or whatever the terms of the lease is.
In a simple system, the barter note may simply be a ticket that never gets traded, merely issued and used. In a slightly more advanced system they trade but only at par value. A gigabyte is a gigabyte regardless of whose server its on -- this is a very simple proxy measure that excludes service quality from automated consideration. Since this is a nanomarket, there is normally no opportunity for the user to intervene with a more sophisticated or subjective judgment. Even a crude proxy measure, if full automated, may be sufficient for a nanomarket to outperform the non-nanomarket status quo (no transactions at all or the use of resource allocation algorithms, although the latter in a broad sense can be considered to be competing nanobarter systems).
In a more sophisticated system (probably overkill for the purposes of Tahoe) some disk space notes trade at a discount because their backup services are unreliable. Bots "ping" the backup services provided by nodes to gather statistics on their reliability, and then buy reliable and sell unreliable notes. There are O((LN)^2) automated currency exchange products which these bots trade. The mental transaction cost problem caused by having O((LN)^2) prices with LN currencies is thus solved underneath the covers by these automated trading bots. The resulting trades are presented to users, if necessary, in terms of pet currencies, and we can have a large barter economy without the mental overhead of all those prices.
To avoid the transaction costs of thinly traded markets, the bots might come to prefer the notes of one or a few services as "intermediate commodities" asMenger described, and most of the markets might become unused, leading to O(LN) actively traded markets -- an economy with a constant number of currencies and LN prices. But that's an entirely optional process that can be allowed to emerge. And with the right reliability-ping and arbitrage bots I suspect the transaction costs of thinly traded markets might be quite small, so that there is no compelling reasoning for a centralized currency to emerge and the added reliability of multiple currencies can be retained without the hassle (mental transaction costs) of users having to deal with multiple currencies.
There are few computational transaction cost barriers left to developing nanotransactions -- the biggest is network delay time. The largest remaining barrier to nanomarkets is, for most kinds of transactions, mental transaction costs. User audits of nanotransactions cannot be both frequent and at fine granularity, or mental transaction costs quickly come to dwarf the value added by the market. Any problems with nanomarkets that might require such audits must be handled in a highly automated fashion.
The approaches to designing this automation all seem to start with developing reasonable proxy measures of service value. For nanomarkets it is far more important that these be measurable in fully automated fashion than that they be terribly accurate. After good proxy measures have been developed, one must obtain or estimate user preferences in terms of these measures. Obtaining preferences directly from the user has to be done in with traditional value granularites, otherwise mental transaction costs dominate. Alternatively, further proxy measures can be made that estimate user economic preferences from their normal input behavior. These are then compiled into automated nanotransactions with the market translator.