DannyHamilton it is impossible for a transaction to occur over the network involving less than a further - tenth of a "femtoBTC (1 fBTC = BTC0.000 000 1)".
The smallest possible "atto" 1/100 millionth unit is called a "sitoshiBTC (sBTC) = BTC0.000 000 01 = 1 sitoshiBTC. The blockchain network cannot record nor verify any smaller of a transaction. BTCitcoin will have a vast, nearly infinite universe of "SitoshiBTC pennies" to use should it's valuation ever need some.
This is true at the moment. The current protocol as implemented in the reference client is unable to represent any value less than 0.00000001 BTC in the blockchain.
However, I think it is rather short-sighted to assume that for the next few hundred years, there will be no possibility of engaging in commerce at values smaller than 0.00000001 BTC.
With a consensus of users, the protocol could be modified and the blockchain could represent smaller values.
It might also be possible to create a new transaction type that represents smaller values.
But even if it is never technically possible to represent values smaller than 0.00000001 BTC in the blockchain, it will still be possible for service providers to offer accounts that represent smaller values.
As a simple example, I am not aware of a U.S. currency (bill or coin) valued at less than $0.01 and yet my local gas (petrol) station charges prices per gallon of $3.949.