You just cannot go wrong with the 17-40mm. While slower and a little less range than the 17-55, it will make the jump with you when/if you go to full frame and you will not be disappointed. That said, what about looking at some fast primes? For about the cost of the 17-40, you could pick up a 35mm f/2 and a 50mm f/1.4. I would think about what you think is technically limiting your creativity at this point and buy to compensate. If your 18-55 lives on the camera but you just aren't happy with the results, buy a mid-range zoom. If none of your lenses function well enough in low light, fast primes.
+1. In the end it's all about personal preference and how you shoot (and sadly what copy you get of each lens)... I've tested sigmas 16-50, tokinas 16-50, canons 17-55, and canon 17-40... sigma and tokina were surprisingly horrid in my copies I tested at the camera store... Soft Soft Soft. the 17-55 was very nice, sharp, and had nice features, however with construction, color, etc... I dont know, for me, I was left wanting a tad more even tho I couldn't quite put my finger on it. For $300 cheaper, the 17-40 had the stronger body, less features, provided sharp, consistent images, and got even sharper when I moved from the 50D to the 7D... (the 50D, to me was way too soft of a camera)... Yeah if canon came out tomorrow with a 17-40 IS mark II or whatever then I'd be the first to sell my lens to pick up the new one, however i dont in any way regret my decision to go with my 17-40.