Current 40D owner contemplating upgrade

This is not correct a 17-55 f2.8 lens appears to be a 27-88 f2.8 on APS-C, the f number does not change, put an EF lens on an APS-C body and the f number remains the same, the same light per unit area falls on the sensor from the same lens, you are simply sampling a smaller area of the image, it is still the same image projected from the lens as you would get on a full frame camera.

Only thing is that changes is the apparent focal length simply because you are looking at a smaller section of the image.

edit: sorry I should have made it clear that I meant if you had a 17-55 EF lens it would appear to be a 27-88 on APS-C, such a lens does not exist in the Canon range the closest would be the 17-40 L. the principle still applies f number does not change.