First, the Canon 17-55 IS. This is my at-the-time-of-this-writing always-mounted lens for my Canon 30D. The obvious difference between these lenses is size - as shown in the picture. The Canon is .4" (9.5mm) wider and 1.2"(28.6mm) longer. The weight is noticeably different as well - The Canon weighs 7.6 oz (215g) more than the Tamron.
The Canon and Tamron are similarly sharp in the center at all focal lengths. At mid zoom range, the Canon begins showing better edge performance - becoming much better at 50mm. Keep in mind that the Canon did not perform well on the ISO 12233 chart test - It doesn't seem to like closer focus distances.
The Tamron 17-50mm f/2.8 XR Di II Lens shows less vignetting than the Canon. The Tamron is more flare-resistant. This Tamron lens' 17mm is wider than this Canon lens' 17mm. Of course, the Canon has 5mm over the Tamron on the long end.
The Canon's biggest advantage is the 3-stop image stabilizer that the Tamron 17-50mm f/2.8 XR Di II Lens lacks. If your subject is not moving (or you are), this is a really nice feature. The Canon also features fast, silent USM (Ultrasonic Motor) focusing with FTM focusing.
Perhaps the biggest difference between these two lenses is the price. Each individual must determine if the Canon's advantages are enough to justify the very large price differential.