Have Mountain Lion Messages running but hidden from dock?
10.8 is a bit more strict than previous versions, with respect to what should be a boolean value.  If you change as below, it should work like you want.

&lt;key&gt;LSUIElement&lt;/key&gt;
&lt;true/&gt;


Note that in this mode, not only is an app hidden from the dock, but the menu bar does not display either when the program is active.