This option is disabled by default: Tools > Preferences > Advanced > Browsing >  Show scroll marker
But enabling this option does not seem to show the position of the bottom of the viewport as you scroll? I remember seeing this in action in Opera 9+, but I have only recently enabled this feature again in Opera 10.62
Now that you mention it - I see it isn't working for me either, and I've always had it enabled. Guess I must have stopped using it, because I didn't miss it. I'm using 10.62 and can confirm - the scroll marker is definitely a dead duck for me.
ditto. 10.62's scroll marker is M.I.A. for me as well. Likewise, I didn't notice it's absence.
I switch between Opera, SRware Iron (Chrome), FireFox, and now IE9 that I lose track of which I'm on sometimes. I browse WebmasterWorld almost exclusively on Opera though, and scroll marker is helpful with the long threads. I hope they patch it up soon.