I suspect when typical transactions start to be less than 1 BTC (i.e. 1 BTC ~= $100), then there will be a lot of support for the idea of moving to microbitcoins. Dealing with quantities less than one hundredth is a pain, but so is dealing with quantities of over a million.
there are many currencies that deal in high numbers (jpy? krw? zwd (extreme)?)
but I don't really know of any that go below 1/100th of the currency's basic unit.
with cash it's easy to see a trillion dollar note vs a billion dollar note, but with online transactions the UI should help with the zeros. (eg commas, 'read' the amount, etc.)
the largest possible number base you would have to deal with is a quadrillion which is used to describe all the bitcoins in the universe..