Everyone can see it. It's a way for people to leave messages for you. It's a hybrid between Private Messages (PM) that people cannot see except you and the person you're conversing and the public forums. I typically use PM if I need to ask someone a question directly since they can then reply directly to me and it will appear in my Inbox. With a visitor message, I would have to go back and check.
Thanks I was hoping to use it to extend my profile to give an Australian perspective. My own sticky, as it were. Eg The relative price of the Prius compared to the Insight is very different to the US. And petrol prices and the average wage. I might try writing there anyway and see if I can link to it.
Unfortunately, spammers have put stuff in my visitor messages at least 2x. Seems like this is a new method spamming...