All of the systems currently widely available are what are called hybrid systems, because the on-sensor phase detection alone is not sensitive or accurate enough to work on its own in a wide enough range of conditions. They use the phase detection to get the lens roughly in focus, which then allows the contrast detection to work over a very small range of possible values, and allows the combined focusing to be both fast and accurate. (There are various claims online that the 70D, which uses the new sensor you mention, isn't a hybrid system, but Canon's own specs state it still uses some contrast-detection in live view mode[1], so my understanding is that it's simply doing more phase-detection and less contrast-detection than earlier systems, rather than being 100% phase detection)
As an example of how unsuited these systems currently are to sports or wildlife photography, in the Canon 70D, there is no way to enable proper continuous autofocus when using live view to shoot still images. Tracking is only available in movie mode.[2]
The camera can continuously provide a rough focus[3] (which speeds up final focus acquisition, and is also probably the exact same system used for movie focusing -- see below), but this is not accurate enough for reliably in-focus shooting. (Behind the scenes, this will almost certainly be using the on-sensor phase detection only).
Whichever mode you half-press the shutter in, the camera will then perform a precise focus (behind the scenes, it's almost certainly using the phase detection data combined with contrast detection data), but at that point, regardless of which of the modes you are in, the focus is locked -- if your subject moves closer or further away, you will need to release and re-press the shutter to regain accurate focus. I'm sure you'll appreciate that if you gave this system to anyone other than an extreme amateur sports or wildlife photographer, they wouldn't touch it.
(Note that the 70D does include a 'tracking' mode[4] in live view, but this merely uses face detection to ensure that the focus area is constantly over a face in the image, it doesn't cause the focus to adjust should the camera-to-subject distance alter while the shutter is half-pressed)
When you're shooting movies, of course the camera will track focus, but while it does a pretty good job compared to previous systems, it's very easy to see how bad the system would be for still photos. Take a look at this example footage[5], where it's very easy to see that the autofocus system is not performing anywhere near what would be required for action photography -- even at the default Youtube size and resolution (without needing to view it full-screen), the inaccuracy and big jumps are very easy to see, on a subject that is moving very slowly and predictably. And that's in pretty excellent light, not an indoor stadium or under trees in overcast conditions. (Also remember that that video is very clearly out of focus at times at a resolution of just over 2 megapixels, and when you're shooting stills you're expecting it to be sharp at 20 megapixels)
As I say, I think it's very likely that improvements to the on-sensor phase detection, as well as algorithmic improvements to the blending of the phase-detection and contrast-detection data, will mean that this is the future of action photography, but it's not there yet, and personally I'd say we're probably 2-3 generations away from a system that can track action as well as even a low-end DSLR that uses a separate dedicated phase-detection sensor.
[1] http://gdlp01.c-wss.com/gds/5/0300011965/01/eos70d-im-en.pdf (Page 436)
[2] http://fr.canon.ch/Images/Dual_Pixel_CMOS_AF_Tech_Explained-...
[3] http://gdlp01.c-wss.com/gds/5/0300011965/01/eos70d-im-en.pdf (Page 228)
[4] http://gdlp01.c-wss.com/gds/5/0300011965/01/eos70d-im-en.pdf (Page 233)