Ubuntu doesn't support tray icons by default and further more, the systray whitelist, which could be used to get applications to use the systray, has been removed with Ubuntu 13.04.
This is pretty bad for TrueCrypt, an open source disk encryption application that still uses a tray icon and doesn't come with an Ubuntu AppIndicator, because you can't bring the mount/dismount dialog up once you close the main window unless you kill the application (it closes to the tray which isn't visible).
For this reason, Stefan Sundin has created an Ubuntu PPA for TrueCrypt which has been patched to use an Ubuntu AppIndicator instead of a tray icon.
Install TrueCrypt with AppIndicator instead of tray icon in Ubuntu
To install TrueCrypt with Ubuntu AppIndicator support (PPA available for Ubuntu 13.04, 12.10 and 12.04), use the commands below in a terminal:
sudo add-apt-repository ppa:stefansundin/truecrypt sudo apt-get update sudo apt-get install truecrypt
The AppIndicator exits when hiding (closing) the main TrueCrypt window - to fix this, open the TrueCrypt preferences and on the "Background Task" tab, uncheck "Exit when there are no mounted volumes".
Thanks to recover for the tip!