If I remember correctly, the forum a member was viewing displayed on their profile if you brought it up. I think it even distinguished between whether they were viewing or replying, etc. This went away some time ago when some major renovations were made to the forums--I think when all the profile customization was added.

This is different from being able to see what members were viewing a particular forum while in that forum though; you had to bring up a member's profile to see where they were.