Although this may sound good in theory, it won't work in practice for the following reason:
The forum software can only see what users are doing every time they do something active on the forum, i.e. clicking a link or pushing a Submit/Preview button.
But users don't necessarily read threads right after clicking or even in the same order as they're clicking, because you can open up multiple windows and/or tabs in IE7, FireFox and other browsers.
To take an example, when I read the forums I go to e.g. the Blank Media forum, and then I open up a new Tab for every thread that I'm interested in. Then I might go to another forum and do the same, or perhaps go to the "New Posts" and open up additional threads in their own Tab from there.
And then when I've opened a number of Tabs, I then read them typically starting with the first one opened.
So the forum software will have little to no idea about which thread I'm reading at the moment.