Not a stupid question!
Truth is, I'm not sure, but if you took the same lens and put it on a FF camera and then on an APS-C camera, both sensors should get the same amount of light and require the same shutter speed and ISO. I can't see any reason why they wouldn't get the same amount of light--what they do with the light is another story.
Well, not true because they'd see a different image. The larger sensor would gather more light just by having more surface area, but if you took the APS-C sized center of the larger sensor and compared it to an APS-C sensor, it should be the same, plus or minus some exposure oddities.
Not all sensors are created equal though, nor are all lenses.
Different lenses have different light transmission properties, so sometimes an f/2.8 zoom (more elements) will require a slower shutter speed than a simple f/2.8 prime, for example.
So there are also t-stops which take the f-stop AND light transmission into account to help keep a constant exposure when switching lenses (mostly used for cinematography).
There's a quick article about t-stops vs. f-stops here:
F-Stops Vs. T-Stops Explained - The Photoletariat
Last edited by farfisa; 11-17-2010 at 11:35 AM.