The Sigma 17-70 f/2.8-4 holds up well. It's not a great lens, by any means, but it's a more-than-worthy upgrade from any of the 'kit' Canons, and I think you'll be surprised at how soft (in a good way) the look is at 70mm and f/4, for portraits. I'd say the main thing holding it back from being a 'pro APS-C' lens is simply the base build quality. Again, it's better than some of the older and outdated Canons, but it's not a patch on the other new Sigmas/Tamrons, and the focus ring in particularly is among the worst I've ever encountered.
If you want pro results out of an APS-C body, and portrait shooting is a concern, the best you're going to get now or any time soon is the Sigma 18-35 + a short telephoto. The Sigma 50-100 f/1.8 is another option, though by that point you're carrying around so much weight and value, you may as well have just bought a second hand 35mm body and a general-purpose zoom.
The Tamron 24-70 f/2.8 VC (especially the latest G2) is worth considering for portraits on APS-C. You're basically getting the look of 40-112mm (roughly) f/4.5, which is about as much as APS-C can hope for in a zoom lens. Again, though, by that point you're carrying around the same size and value as a second hand 5D2 and 24-105, so it all becomes a bit irrelevant.
Another f/1.8 zoom or other pro-quality EF-S lens is unlikely to come about now, as sales of APS-C-only lenses are dropping, fast. APS-C bodies as a whole aren't selling as well as they used to, and with the price of older 35mm bodies lowering quickly, people aren't investing in APS-C lenses. That's not to say that it's utterly impossible for a new, high-quality EF-S mount lens to turn up, but it's very, very unlikely. The market just isn't there anymore.
For what it's worth, if moving to a 35mm body wasn't an option yet I was still putting together a kit for portraits/events where people are the focus, and getting that trendy background blur was a concern, I wouldn't try to get everything done with just one zoom. I'd get the Sigma 18-35 and a Canon 85mm f/1.4, or the Tamron 85mm f/1.8 if the Canon was out of budget. If it had to be one zoom it'd be the Tamron 24-70, though I don't care so much about wide-angle shots so YMMV.