Edit: The answer is that they are unsafe and have been attacked. Please don't use them!
Brain wallets can be created with a (minimum 15 character) pass phrase at https://www.bitaddress.org
There are no warnings given on the site about the safety of this. How secure are brain wallets?
A recent contest provided 5 bitcoin (receiving) addresses generated with a brain wallet pass phrase, with each address holding 1BTC as a prize.
The challenger did give some hints along the way, but within 7 hours, the first prize was taken, and after around 18 days, all of them were taken.
The author of the blog summarises that brain wallets are generally safe, but is this really the case?
With a well indexed database of the existing Bitcoin addresses that hold a positive balance and a well-tuned script for trying out likely pass phrases, surely there's some quite long phrases that would still be easy to guess?