Tom is already aware of this issue and it probably will be updated soon.
He's working on several parts of the website and details like those will be fixed in the process.
Once you noticed it, take the habit to click your nickname in the top right of the screen in the forums, the dropdown box contains a mention of the "Inbox".
It's a workaround until it gets fixed.
<img src="smileys/smiley1.gif" border="0" align="middle" />