Yeah, I noticed that it disappeared one day. I think the forum upgrade might have had something to do with it.
I'll see if I can figure out what's up.
Edit: Looks like I fixed it. For some reason the PNG icon just stopped showing up. I converted it to an ICO and it seems to be okay now. If it doesn't show up, clear your cache and refresh.
Speaking of that, if anyone can come up with a better one (shouldn't be hard), link it here and I'll change it. I just did that one real quick so that I could have a unique icon to use as a pinned tab.