I've found that the 'getButton()' method on MouseEvent doesn't returns the correct values under JMEDesktop. You get the value minus one.
That is, if you receive a MouseEvent for a mouse click, and you clicked the left mouse button, you might expect that:
mouseEvent.getButton() == MouseEvent.BUTTON1
The MouseEvent.BUTTON1 value is 1. But using JMEDesktop, that 'mouseEvent' instance you get, returns the value '0'. Which on MouseEvent is 'NOBUTTON' constant.
Is a small issue, as looks like the mouse button values are set as they were originally on the input through lwjgl. (I think, unless I'm doing something wrong).
In deed - thanks for finding it out! (fixed in CVS)