"Last read post" function

I visit a few other sites that automatically indicate to the user the last post he read. The site senses where the user stopped scrolling, marks the first unread post, and snaps the user directly to it if he comes back to the thread. This is very handy, I've found. It would be nice for backreading long old threads or for people who take rather longish breaks now and then (like I have done). I don't know if those other sites use the same language as this one, or if such a function would be possible here, but I think it's worth suggesting. Perhaps on an optional basis.

So "last read post" is determined by how far you scroll, not the date of the most recent post in the thread you're viewing? Sounds a bit like Google Reader... I'll hunt around for something like this, but on the basis of preliminary Googling, I doubt an out-of-the-box solution for our setup is in existence.