Hi, Rachy, and thanks for posting.
You're right - the button has disappeared. I removed it at the weekend as part of a general tweaking of the forum index template to accommodate forum sponsors' banners.
You can still get the exact same function, though - by clicking on the "View unread posts" link that appears at the left of the page header, just below the "Log out" link. The only differences between the button and that link are that (a) the "View unread posts" link appears on all pages, not just the forum index page; and (b) the link text changes to "You have no unread posts" when there's nothing unread, whereas the button used to appear whether or not there was anything new (which I always thought was a bit pointless and annoying).