There's no icon of dropbox in the gnome-shell's tray (bottom-right corner). There's just text. I tried different icon themes but still it's missing. Here's a screenshot:
I had to reinstall Dropbox due to some problems with memory leaks. That has been solved, but I've noticed that the icon that used to appear down in the bottom right is now gone.
Opening up system activity verifies that Dropbox is indeed running.
I also have the Unity shell on my computer, and switching to Unity displays the Dropbox icon.
I just installed Dropbox, and it put a permanent notification in my notification tray. The only way I can get rid of it is to quit Dropbox.