Can you elaborate on this please?
Yeah, sure:
(I'll try to explain it in simple terms, please don't ask me about the specific math/cryptography behind it
)-A bitcoin address is more or less a public key of an asymmetric crypto-system.
-A wallet can be seen as the private key of this crypto-system.
-To generate a signature you need to have a whole pair (public AND private key). Same as if you want to send some bitcoins that lie on a bitcoin address!
-The generated signature is unique for one public key and the message that was signed (e.g. for one bitcoin address and a message)
-Anyone can verify whether message, signature and public key fit together WITHOUT having the private key
In the OP of this thread you'll find the bitcoin address containing the shares, a message and a signature. I wrote the message (made sure I wrote my nickname in it. Otherwise anyone else could take message+address+signature and say it was his^^) and signed it with the shares-address via my wallet. No one else could have generated that signature because no one else has the private key to my wallet. You can now open your bitcoin client and verifiy if address+message+signature fit together and thereby verify that the address is controled by user "acayne".
Hard stuff to explain, I hope I found the right words.
You can only sign a message with a given address if you indeed own it/own the private key to it. 
