I've done basically what you've described
with SmartMacros and it works flawlessly.
I created a Lamp On macro, a Lamp Off
macro, and a Motion Detected macro. The
lamp comes on (with a timer) at dusk (at
50%) and off (with a timer)at dawn. The
Motion Sensor triggers the Motion Detected
macro that causes the lamp to 100%
brightness for 20 seconds and then back to
50% brightness. Hope this helps.