Since we're having a currency debate.
I believe that currency is useful as long as it has some sort of material backing it up. I don't believe in trading a material for a set price for another material, neither do I think bartering is very efficient.
I think Notch should implement a system so that you can design your own print on paper. This would allow for regional currencies and for regular paper to be used as money instead of useless books. This would then be backed by the amount of material that person has, similar to a bank note.
Here's how it would work:
A town would be founded in a SMP world, slowly it would be built up and ores would constantly be found in their community mine. Hopefully someone builds a bank with sort of vault underneath. Now here's where the town's government comes in, they would pay the person who owns the bank for space in the vault to store gold. They would then commission the owner to print notes to represent the gold in the vault. This currency would slowly be spread by the act of bartering, and circulated through the town.
Slowly the currency would loose value and the government would have to intervene with more gold, and so on.
Rough idea.