Both those lenses are excellent. The 17-55 likes to flare when a light source is in the frame, so a hood is a must. Other than that, they both have great AF, IS and produce equally awesome pictures. So I think it's a matter of what you'll be using it for.
The 15-85 is ideally suited for sightseeing, and outdoors. It's usually bright outside, so a narrower aperture is fine and this lens is much more civilized when it comes to flare. It also has more reach, and a little more breadth.
The 17-55 is much better for indoor/event and evening shooting.
I love having both, but I only ever use one at a time, depending on where/what I'm shooting.
If I could only have one, it would be the 15-85. I find I can never have enough focal length, and I'm willing to up my ISO a stop or two to get the shutter speed I need.
No, I'm not aware of any updates to the seals on the 17-55. Yes, an f2.8 lens needs only half as much light as an f4. That means you can use a 2 times faster shutter speed or a stop slower, less noisy, ISO. Plus the f2.8 will give you more background blurring which is very nice for portraits where you want to isolate the subject from the background.
I did a quick review of lens dust threads on the PhotographyOnTheNet Canon forums and found dust threads for pretty much every Canon lens that extends when zoomed as well as a few other third-party lenses that aren't considered inexpensive. Interestingly, none of these threads actually mentioned finding a degradation in image quality which lead to the discovery of internal dust but, rather, were started by someone who inevitably looked through the lens with a flashlight, saw some dust and grew concerned. I'm of the philosophy that if it ain't broke, don't go looking for problems. ;) (When I last bought my brand new car, I made a very conscious - though difficult - effort not to look for scratches in the paint after every trip to the grocery store.)
The general consensus is that SOME amount of dust in the lens will not affect image quality whatsoever. Many owners of the 17-55 also report that installing a front UV filter has kept their lens pristine (I've also kept my UV filter on at all times except when using a CPL filter so perhaps that isn't an unreasonable conclusion). Apparently there are three small hidden vents on the front of the lens a filter will seal. Furthermore, one of the long time members of the forum has published do-it-yourself instructions for internally cleaning the lens and others point out that Canon will give you a free (minus postage) cleaning and adjustment while the lens is still in warranty.
I guess what I'm trying to say is that it really isn't something worth worrying too much about under typical conditions.
This lens is not a sports lens. You'll be too far from any action to even when cropped. The 70-200 f2.8 is one of the best sport lenses for up close. If you want even closer I'd recommend buying the 300mm or 400mm but its might be easier to buy a tube extender that will increase the range of your lens. This lens is great for landscape, portraits, and Weddings.
The real problem is that the 17-55 f/2.8 lens is close to what every one wants but not quite there so people are disappointed with it. What dose every one looking at this lens really want? A real standard zoom L lens for ASP-C Bodies. Something with the same build/optical quality and features as the 24-70 f/2.8L, but with a focal length range that provides the same coverage on a ASP-C body. Something like 15mm on the wide end and like 45mm at the telephoto end. Since this lens doesn't exist people are forced the buy the next closest thing. Which is the EF-S 17-55mm f/2.8 IS.
Quite frankly for the build quality the 17-55 f/2.8 is way over priced and already in the same price bracket as an L lens. Which is why people expect same quality and then are dissatisfied when the build quality isn't much better then the kit lens. I don't see why canon doesn't just make an L version of the 17-55mm f/2.8 and sell it for like $1200-$1500. Especially considering the existence of the 7D, which is basically a pro-level ASP-C body. Oh wait I know, It's to force people who want/need to have L lens quality with decent wide coverage to upgrade to a full frame camera.
Short answer - physically it won't fit because the back end sticks out and will hit the mirror resulting in a broken lens.
Long answer - The reason the focal length is given as such is because this lens is ACTUALLY 17-55mm. The fact is that part of the image gets chopped off by the smaller sensor. Imagine a rectangle with a large circle on top of it. The circle represents the projected image and the rectangle is the sensor. Some of the projected image gets chopped or "cropped" by 1.6x. So now that 17mm angle becomes narrower by exactly 1.6 = 27mm. So it started off as 17mm but by the time you get to see the final image it is more like 27mm.
Quite simply if you could glue this lens onto a full frame camera somehow (which I DO NOT RECOMMEND) then it would actually be a 17-55mm lens.
If Canon were to write the cropped value it might confuse people when they mount a non EF-S lens. You would have 2 naming conventions and things could get messy fast. It's easier just to remember that you are using a crop sensor and multiply everything by 1.6.
Actually, I'm merely guessing on the focal length from your inquiry about the 17-55mm. Your choice will depend on how much reach you actually need.
At $80, the Canon 50mm f1.8 is very inexpensive with pretty good optics and a wide aperture. Focusing may be too slow for your needs but its so cheap it might be worth a try.
The Canon 50mm f1.4 adds fast USM focusing and somewhat better optics.
The Canon 85mm f1.8 is a good choice if you need more reach.
The Sigma 30mm F1.4 EX DC HSM get pretty good ratings and is a good choice if you're fairly close or need a wider view.
Tom covers this well, but I'd add that you'll want to select the center AF focusing point only...don't select the whole array. I was irritated by focusing issues on my new Canon 7D until I went back and checked the focus points. Sure enough, on large-aperture portraits it was picking out the nearest points first and not providing enough depth to keep the whole cubject in focus. I switched to running Av only and with the ceter focusing point only. Much better results.