that sounds like a good idea am concerned that it might potentially clutter the interface if there is a checkbox next to each thread with "mark as read".
in that case maybe the option could come up in the dialog box when you long press the upvote and if checked the icon will turn orange or maybe if we had to double click in order to upvote so the first click is just you marking it as read but the second click gives a sat or whichever your default is. or something even better :) doesn't have to be perfect i guess