Sometimes, after kernal updates in Ubuntu the Dropbox icon is removed from the panel. For me, it is nice to see the cycling symbol on the icon as files and folders are synced. In any event, here is what I found to get it back.
From the terminal:
$ sudo apt-get install dconf-tools
Then in the dconf-editor dialog:
desktop -> unity -> panel
From there, add an entry 'Dropbox' into the systray-whitelist value field.
Mine looks like: ['JavaEmbeddedFrame', 'Wine', 'Update-notifier','Dropbox']
Finally, log off and back on or reboot.