OK then
I have used the both the Canon 10-22 and the 17-40L on a 50D and a 7D. They are undoubtedly both decent lenses in their own way. But what you truly want to use the lenses for is going to help you decide more clearly. Be as honest with yourself as you can as you are going to shell out up to £615 (best price I could get for the 17-40L with quick google-fu) and no doubt want to get the most use out of it. Having read some of your posts re your travels/adventures etc I can see why you would want a true wide angle in your kit 50mm doesn't cut it on a crop (80mm eq. is a decent portrait lens but no vista taker). Which brings me on to the wide angle. 17-40mm on your 50D is only 27-64mm which is wide enough to get a decent landscape but not IMO sufficient enough to get as creative artistically in a studio situation/indoors with portraits or still life. The L lenses are quality lenses but the 17-40L is not a new lens...nearly 10years old. None of the lenses I mention will have IS but it isn't that necessary with a constant f4 aperture in this case (and being a wide angle lens). It's bigger and heavier than other similar lenses, but TBH for an L class lens it's quite lightweight and you are getting very good glass and it is weatherproof. I have used one in dry/arid desert locations (rather than wet) and it coped well with the dust (plenty of reviews mentioning water), plus it is pretty robust and will take a few knocks. So, big, heavy-ish, no IS, oldish design and not the widest, but quality glass with weatherproofing, fast-ish (for a w/a zoom)
Next up, the Canon 10-22mm...IMO about £150 - £200 too expensive and a lens bought by Canon fanboys without doing much homework. Sorry, sounds a bit harsh, but for the money (at circa £600-650) it is not as good as two, possibly three alternatives. Before I go on any further, I should say that I am by no means the de facto authority on photographic equipment. I use cameras in my daily work for a multitude of tasks, but the equipment I use really gets used. As a company we have never seen the point in spending money on anything that wasn't tried and tested so we usually hired gear before buying, especially when it came to glass. Getting back on topic; this lens is one of those items that Canon has in it's line-up that carries a price tag because of the name on the case. I don't think Canon (Photographic) is as guilty as some other companies (can you hear me up there Steve Jobs) but this lens is one for the pot. It's a decent lens, IMO it has a more creative range of 16-35 eq than the 27-64 eq on the 17-40mm (getting complicated here) but other than that it's no better than what follows...or arguably not as good (many conflicting reviews online, but IMO the lens is beaten on price).
I would therefore suggest looking at the Sigma 10-20mm F3.5 EX DC HSM. Constant F3.5 aperture as good build quality as the Canon and IMO better pic quality even wide open. Great fast-ish glass for £200 or so less. We had this at work and it was one of our favourite lenses. I have used this a lot with great results, it's well built (I have had several Sigma lenses along with Canons) and comes with it's own case and petal hood. Main drawback is the massive 82mm filter thread!
I have the older Sigma 10-22mm F4-5.6 and wouldn't go on any shoot without it. Not as fast but rock solid build and good quality pics.
The other option would be the Tamron SP AF 10-24mm f/3.5-4.5 Di II LD Aspherical (IF). I have heard good things about this lens professionally, but I have not used it myself...although, personally I don't like the aesthetics...but is that a reason not to buy..don't know.
So, to sum up, if it were me, I would be looking to buy either the 17-40L or the Sigma 10-20 F3.5. There's a sizeable difference in price, but also in their uses and capabilities. You will have to work that one out for yourself, but if you want any other input just let me know.
Sorry, long post, but with regard to what Wayland said re the gaps in your lens coverage (which is sound advice especially if you want/need to get full coverage..but), I'd say overkill unless your livelihood depends on that exquisite perfectly composed shot for which you are prepared to hump miles with 100kg of equipment. Having read your bio I would think that you know enough about equipment and photography/videography to make an informed decision, but FWIW I would suggest that you tailor your gear/glass per trip. Waylands photos are superb, but under the circumstances that I have taken most of my pics I have usually had the 'luxury' of having 2-3 lenses max and 2 bodies.
ATB
Sack