Just some interesting math: the possible range of BTC values is between 0.00000001 to 21,000,000 (https://en.bitcoin.it/wiki/PHP_developer_intro#Precision).

In theory if BTC becomes popular enough (and thus increasingly valuable, thanks to a fixed maximum) it will be traded in uBTC (0.000001), so people would treat uBTC as "dollars", and that still leaves two decimal places for cents, i.e. you'd be buying things for 123.45 uBTC. In uBTC, with the maximum value of 21,000,000BTC, there's an in-built currency limit of

21,000,000,000,000.00 uBTC.

By comparison, there is currently 1035.2 billion USD in circulation in the EU (http://en.wikipedia.org/wiki/Circulation_(currency) ), which is:

1,035,200,000,000.00 USD

So if necessary there is 20 times more uBTC than there is money currently in circulation in the EU.

EDIT - changed mBTC to uBTC