I would suggest the Canon 17-55mm f/2.8 IS USM lens as it is truly designed for the camera body that you have, and almost perfectly fills the focal length that you are looking to fill in.
It is an excellent lens, and gives an aperture of f/2.8 that is unmatched in any lens that also offers image stabilization in this range. This lens is a reason that many photographers hold on to a crop sensor(APS-C) body such as the Canon 7D even when they have full frame available to them.
I know that you suggested you may end up buying a full frame camera soon, and this lens will not be compatible, but I would suggest buying equipment that suits your needs now, and worry about upgrading when you are ready. The amount of money that you will lose when you go to sell this lens is comparatively small to the amount of money you will be spending to buy a full frame body anyways.
The Canon EF-S 17-85mm f/4-5.6 IS USM is also a similar lens in this range, but not quite up to the quality or maximum aperture of the lens listed above.