I second that motion! : ) OnMouseDown would be very helpful with making more realistic buttons. It would also be nice to be able to implement several other events as well: onmousemove, onmouseout, onmouseover, onmouseup, onwheel.
At first I thought about submitting a bug report. Because when I first started playing with Colludium's tutorial, I tried to use Mouse plugin events (mouse button is down, on button released) as a work-around. But found that the mouse plugin events don't play nicely with the button plugin. If the mouse button is pressed down over a button, but then the mouse is moved off the button object, and then the mouse button is released, the on button released event does not fire. So, if you move the mouse back over the button object, then it was acting like the mouse button was still down (when it wasn't). But, just using mouse events within the button plugin fixed that issue - but meant I had to hack the button plugin to add the onmousedown trigger.