If you look for them, there are dimmable compact florescent lamps available. These at least hold much of their efficiency when dimmed.
Incandescent lamps are interesting beasties. They are resistance heaters, but the resistance changes by a factor of 10 or so between a cold filament and a hot filament. A 100W lamp will draw on the order of 10A (1200W) at startup.
There are a couple of approximate equations that describe how the light output changes with input voltage. Because the resistance changes with temperature, and the temperature changes with power dissipation, the normal V^2/R equation derived from Ohm's law doesn't really work.
The power consumed by a lamp is proportional to V^1.5
The light output of the lamp is proportional to V^3.5
The life of the lamp is proportional to V^-13
If you increase the voltage to a lamp by 10%, the power consumed will go up by 15%, the light output will go up by 40%, but the life will go down by 70%
If you were to compare a single 100W bulb to the output of 2 100W bulbs run at 50W each, the 100W bulb would produce about 2.5x the total light output of the pair of half power bulbs. A 100W bulb so dimmed will consume 50W of power, but will produce the light of an undimmed 20W bulb. Less power in, but far less efficiency; you are better off with a smaller bulb unless you like the color.
The dim bulbs also have much longer life, which can be a signficiant benefit.
iwire: you might consider looking at some of the LED bulbs; some are rated for 120V, and probably some rated for outdoor use. There is quite a bit of BS in the market, where the light output gets inflated in the advertising but not in service... but if you only need a little bit of light they are great.
-Jon
