In theory there's no limit to how much a bitcoin can be divided. Currently 0.00000001BTC is the smallest amount possible. But if there's only 0.00001 bitcoins left in the world then the program can be easily updated to allow 0.00000000000000000001BTC to be sent and used. Bitcoins are a bit like gold, you can just keep dividing a bar up smaller and smaller into dust. Of course if all bitcoins in the world vanished after the last block (with a reward) had been mined then that would be the end of bitcoins but that seems pretty unlikely right now.