Go with what's best for you now. I had the same dilemma, do I avoid all ef-s lenses just in case I upgrade, or do I get the best quality and most sensible zoom range for the camera I have at the moment?
I went with the 17-55mm having also had a 24-105mm for a month (borrowed from a friend to help me decide). I'm completely happy with my choice
, the range this gives you on a crop is excellent and the 2.8 combined with IS make it outstanding for indoor shooting. The images are also sharper than the 24-105mm (in my experience) and 24 isn't wide enough.
I checked over the used prices of the Canon lenses when I started buying gear and they hold their value really well so my plan is that if I ever upgrade to FF I'll just sell my ef-s lenses (I have the 10-22mm and 60mm Macro also) and get some EF equivalents. I've recently upgraded to a 40d from a 400d and the lure of FF isn't really there like it was, the high ISO performance of the 40d is great and maybe if the replacement for the 5d brings really revolutionary changes then I might want to switch again, but I doubt it. You also have to consider the size of the prints you do, I had a picture from my 400d at ISO1600 printed at A3 and framed and it looked great, FF will let you go bigger than crop but you have to go pretty big.
However, I've found the 17-55mm such a good all-round lens that I think I'd want to see something like the 24-70mm F/2/8L get IS added to it before I changed to FF and even then it would be tough to part with the 17-55mm. You might read on some forums that people say that the 17-55mm build quality isn't on par with the L lenses, and that it can collect small amounts of dust inside. But from my experience having had the lens for about 6 months, I have no dust, and yes the feel of it isn't as solid as my L series lens but it's not cheap or shabby, far from it.
My vote is the 17-55mm.