I have spent quite a lot of time (~250h) on this problem as part of my dissertation and found ~18k brainwallets. Most of the 10k brainwallets found by the author have probably been made by another 'researcher' who is actively probing the network to look for thieves. There are many other similar analyses online which have better and more interesting results than this.
Edit: I can upload some rather large and confusing transaction network diagrams if anyone wishes to see them.
I have a few more but they are even more confusing.
Red nodes are brainwallets and blue nodes addresses that brainwallets transfer to. You will notice the massive cluster on the left, that is the 'researcher' who is actively probing the network.
The rest are standard brainwallet transactions and thefts.
I think the cluster at the end of July/beginning of August of 2013 was me. I am not entirely sure because it was during BlackHat/defcon and I was drunk. Was a few thousand sampled from the rockyou list? Post a pgp key I have a question for you.
Edit: I can upload some rather large and confusing transaction network diagrams if anyone wishes to see them.