I used to have the 17-35/2.8L instead of the 17-40L.... But sold it to get a wider lens for use on crop. It wasn't anywhere near wide enough, so I replaced the 17-35 initially with a Tokina 12-24/4 and now also have Canon EF-S 10-22mm.
I would jump on the 17-40 as a great lens and good value for use on FF. But not on crop. I'd go for the 17-55/2.8 instead. This is a mid-range, walk-around lens on crop (slightly wide to slightly tele) and f2.8 is too important. IS is a bonus. Not necessary, but nice to have.
I wouldn't worry at all getting a quality EF-S lens now, if thinking about "going FF later". I used to think that way, too, but no longer do. For one, you can always sell the EF-S lens, if and when you do switch to FF. With higher quality Canon EF-S lenses like the 17-55, reselling you would likely recoup at least 75 to 90% of the original purchase price. If you've used it for a few years, you might break even or make a profit (several of my lenses would now sell used for more than I paid for them years ago). Plus, you might never find the need to "go FF", crop cameras today come awfully close to FF image quality. Or you might find yourself using both formats (I do) and end up wanting to keep a couple "crop only" lenses in your stable.
Not that the 17-40 is a bad lens. It's actually quite good for the money... on FF. IMO, on a crop camera it's not wide enough or long enough or fast enough. There are better choices available for crop, so why hobble yourself for the years you use the crop camera? Besides the EF-S 17-55 IS, as others have noted, there are some excellent third party options.