It is possible to brute force some Bitcoin addresses, because some people generate their private keys in an insecure manner. Any (non-zero) 32 bytes can be a private key. So running sha256 over a passphrase gives an apparently random, but brute force-able private key.
Take sha256("sausage") for instance:
$ echo -n 'sausage' | sha256sum
Load up bitaddress and paste that private key into the 'wallet details' tab to get the corresponding Bitcoin address, then look it up on blockexplorer:
$ GET http://blockexplorer.com/q/getreceivedbyaddress/1TnnhMEgic5g4ttrCQyDopwqTs4hheuNZ; echo
and you'll see that the address held one bitcent for about 2 days in February 2012.
See also: "fuckyou", which held 2.5 bitcents for 12 festive days at the turn of last year.
So in practice it's possible to brute force bitcoin address creation, but only for poorly chosen passphrases. These were probably just people playing around with the idea of "storing bitcoins in their head" which is why they are for such small amounts, and why they weren't left funded for long.
No address balances were harmed in the making of this answer.