Thanks for the info, Crossroads. The top scheme did work!
... But I am still confused as to what was happening with my MOSFET. I thought that you could use them in place of a relay in a situation like this. I wanted one digital pin to control wether or not the LDD-600H was getting power or not, and the PWM pin would control the brightness. When I had teh driver hooked directly to the power supply the errant voltage in the "off" state wasn't there, so I knew it was some issue with the MOSFET I was not seeing.
Even though the light works now, I can't move on till i know what's going on!