My situation is thus.... Our bedroom has a tray ceiling with four dimming spot lights near each corner. We are going to install a fan (prossibly with no light) in the middle of the ceiling and want to keep the lights on a seperate switch. I can get above the ceiling to add a support box and run a wire for the fan down to the current light switch in the wall.
By "tray ceiling" I assume you mean a dropped ceiling with ceiling tiles?
That switch will work, except instead of both wires coming off the switch and going to the fan box, the light dimmer would go to your existing lights, and the fan control would go to the fan. You could put a light on the ceiling fan and control it from the same light dimmer (provided you don't exceed the rating of the dimmer). Usually the dimmers are only rated for about 300 watts, so you will have to check; also will depend on the wattage of the bulbs you are using.
To make that box accept 2 switches down the road, you could simply cut the drywall big enough for a 2 gang box, rip out the old single gang box and put in an "old work" 2 gang box feeding the existing wires into it.