One Bitcoin is divisible down to eight decimal places. There are really 2,099,999,997,690,000 (just over 2 quadrillion) maximum possible atomic units in the bitcoin design.
The value of "1 BTC" represents 100,000,000 of these. In other words, each is divisible by up to 10^8.
As the value of the unit of 1 BTC grows too large to be useful for day to day transactions, people can start dealing in smaller units, such as milli-bitcoins (mBTC) or micro-bitcoins (uBTC).