For others who might wonder, the answer is that the camera's imager is native 720, but has internal "up rez" processing to 1080 so that recording is 1920x1080 (but horizontally down rezzed to 1440). It will nor result in as sharp a picture as woulde be created in a native 1080 camera (like the Panny 3000/3700 or Sony F900/800/700).
Horizontal stretching is nearly impossible to see. The images are excellent and if you saw the two side-by-side, you'd be hard pressed to tell the image difference. I've mixed both images together and depending on who shot it, sometimes the Panny looks better.
In 1080, HDCAM subsamples horizontal resolution to 1440, DVCPRO HD to 1280. The latter is a 3/4 subsample, using rectangular pixels, HDCAM also uses rectanguler pixels. HDCAM runs at 135Mbps, using 7:1 compression, 3:1:1 color space, DVCPRO HD runs at 100Mbps(less at Native frame rate modes), 6.7:1 compression, 4:2:2 color space.
The new XDCAM 4:2:2 format runs at 50Mbps, 4:2:2 color space, Long GOP frame structure, full horizontal sampling with square pixels, 8-bit, like HDCAM and DVCPRO HD.
AVC-Intra 100 runs at 100Mbps(but actually lower for most frame rates since it is a progressive, Native codec), 4:2:2 color space, I-Frame structure, full horizontal sampling with square pixels, 10-bit depth, giving it 4X the quality of 8-bit formats as far as shades of gray.
The native 720P CCD's found in the HDX900, Varicam 27H, HPX2000 and HPX2700 have a very nice look and I find them to be virtually indistinguishable compared to a full raster CCD chipset, with the advantage of better sensitivity(comparing HPX2700 to HPX3700, as an example). This is live out of the 10-bit HD SDI output, once recorded on DVCPRO HD, it's really hard to see any difference, or even AVC-Intra 100. To get the most out of a full raster camera, the lens quality has to be excellent, beyond that of most HD ENG zooms.