I got the invalid token message when I logged in. I get this on loads of my sites too. Since I don't know the solution I can't help. But if you do find it I think it would be a great help to your users. I don't doubt that this is a Joomla! issue, but I note a lot of forum posts regarding this issue, and having searched the Joomla forum, I don't know of a solution. I thought you guys might have solved it, as I'm fairly fresh behind the ears.
Yah it's a Joomla security thing. Basically Joomla uses a session based token that must be passed into each form submission. That token is associated with your session, so if you have a form open, for example a forum post you are writing, for a long time and your session times out, you will see this error when you try to submit.
There are workarounds that involve using 'keep-alives' when your on a form page, so if you let us know where you got this invalid token error, we'll take a look.