Having noted the shortcomings of the Tamron 17-50 VC, the pixel-peeper in me would not let me overlook them.
So, having made a conscious decision to let it go, the options remaining are the Canon 17-55 and the new Sigma 17-50, which both look very good indeed. The Canon has, of course, the advantage of being Canon, full compatibility with past and future EF-S cameras, while Sigma might need a firmware upgrade to work with new bodies some years from now. But to be honest, that doesn't bother me so much. The Sigma is also a bit shorter, a bit lighter, and comes with a lens-hood.
While the Canon has higher scores (and equally higher price), I am somehow drawn more to the Sigma. While I know it would most likely be a bit better, there is something about getting too many components from the same manufacturer that I don't like. There is something I find appealing in having parts from many different sources combined; I also felt this in something so simple as getting the new Nikon-branded lenscap for the 50mm... It's probably also the reason why I don't feel any need or desire to get the Apple-branded keyboard and mouse, instead going with Logitech, and using an LG LCD.
But both the Sigma and Canon are way out of my price-range at the moment. So, wanting something at least a bit better than the old 18-55 kit, I got an 18-55 IS. This might not sound like such a big change, but the new optical recipe Canon had for it is a real winner according to almost all reviews.
Here are some images I took recently, trying out the improved close focusing.
Update: after doing some pretty extensive testing, I have noticed that my old 18-55 is actually sharper than the new IS I bought. This runs opposite of pretty much everything I've read about it, so I'm suspecting I got a bad sample. Overall it's still a nicer lens, with IS, better close focus and nicer feel, but it seems I can't accept the step-down in image quality, no matter how slight it is. This means I don't have any reason not to start saving for the Sigma right away...