After holding out for a full two years (since 11.04) when Unity was made the default Desktop Environment, what was promised to happen in Unity with relation to the legacy systray has final happened. It's dead. Door nob dead...
Well, sort of.
As of the latest update to Unity (today, Feb 7th) in 13.04, the system tray (and the dconf value that enabled it) are no more. Unity will as of now only display application indicators, which means greater consistency, but in some cases, less compatibility. For example, Guake, the drop down terminal that so many love, no longer shows a system tray icon in Unity even with the system tray having been enabled before. However, this will eliminate some minor niggles that occurred in the past with the system tray when it was enabled (and even sometimes when it was not), such as the phantom icons that appeared in expo view.
Developers who previously used the system tray for their applications can easily add support for application indicators (and they should). This will give their application a modern experience in Unity, KDE, and... to some extent, Gnome Shell.