Ohh, NOW I see it!
Thanks for blowing up the image.
Just checked, and I have that too (Vista Home Premium x64). My laptop (same OS version) has it, as well.
I'll bet if we got to talk to the Vista design team, it's intentional and it has to do with some sort of 3D or shadow-type effect, like the "convex" taskbar. But if you're on Vista and you're not getting the line, and you haven't customized your visual effects, then my hypothesis is suspect.
Funny, it's the thicker line that I mentioned, which goes up only about two-thirds of the way instead of the whole length, that makes me scratch my head.