Do you have any Problems sending Messages trough iMessage Beta for Mac?
So I did and found a Solution. It seems some DNS servers are not working well with this.
Try to change the DNS Server in you Network Preferences:
In System Preferences <> Network > Advanced Button > DNS tab add the DNS server 220.127.116.11 (it’s a Google Public one)
After editing the DNS Server under Preferences in Mac OSX iMessage works fine but my Mail doesn’t work anymore.
I found the Problem: Had to change the DNS Entry on my router too!