Of course there would be an opt out like discussed before. you could have your thread effectively be private so that you don't see who is reading your threads...and I'd think when you're hidden it shows as +1 hidden or whatever so if you were invisible and someone checked who read their thread, it would show up somethin likeRecently viewed by...
+3 Hidden <---example (those who are invisible) or...
Maybe put an addition option in your profile that says something like "Do not show me on thread history."
How' that eh?
I'd just either extend the session table (eh...) or create a new one that tracked thread visits accordingly. Then you tack on the appropriate code to thread display to update (and prune) the table on thread view. You need to add the thread's result to $context as well, so that the theme can display it.
In principle, it's easier than the ons and offs deal I'm building.
So if it's easier, in principle, would you be in favor on working on this feature as opposed to what you are building now?