Keep in mind, however - that even if you purchase the 24-70 2.8L from Canon, you'll have to temper your expectations somewhat. I went from a Tamron 24-75 2.8 to the Canon version, and to be perfectly honest, the Tamron IS just as good a performer as the Canon in terms of sharpness. Once I switched to the Canon 24-70, it was like moving up from a Camry to a Lexus. Small changes, to be sure - but they're changes you'll either appreciate or think frivolous.
In all honesty, the only reason I bought the Canon version was for the weather sealing and for the fancy red ring denoting L glass. /shugs. It's a very good lens, but I wouldn't overlook the 3rd party variants either - they're VERY able performers.