I had Implite click on "preview post" in the same thread as me to help me with this site suggestion.
Above image, you can see that I am in the thread (along with 3 others. 1 being Anonymous.)
When you hit "Preview Post", instead of you still being in the thread, it makes you the "Anonymous" user.
As an S/A helper, It makes it hard to see if another S/A helper has been working on the thread and is in the "Preview Post" section.
(We were talking via Ventrillo while making this Site Suggestion so the time is a couple minutes off but we were both still in the thread)
Above image is Implite in the Preview Post section.
In the Preview Post Screen, you cannot see if anyone is viewing the thread as well.
Special thanks to Matt (Implite) for helping me with this!