As of Eclipse 4.2, it is not well documented how to implement a status bar.
There seems to be an improvement over E3 though, that the status bar is no longer tied to a view
and thus can display messages from async jobs etc.
http://www.eclipse.org/forums/index.php/m/962170/ (StatusLineManager
http://www.vogella.com/articles/EclipseJobs/article.html
http://www.eclipse.org/forums/index.php/m/753670/
Note that "@Inject IStatusLineManager" (as described in http://wiki.eclipse.org/Eclipse4/RCP/FAQ#Accessing_the_status_line and elsewhere)
does not work, see also https://bugs.eclipse.org/bugs/show_bug.cgi?id=332499