assuming that you run a bitcoin client on the pc that you're using at the moment and you want to unlock it to send a tip.
i dont think you can assume it for most of the cases.
and someone may not want to wait 6 confirmations to get his tip from an unknown sender.
it should be integrated into the forum - this would improve quality of the content.
its much easier and efficient to just click a button, rather than do a bitcoin transaction.