From the Canadian Sienna's forum, in
Electronic brochures for Canadian Siennas, a subject which I don't think is unique to Canada:

Originally Posted by
tcp
In the brochure it looks like the factory hitch has a cutout in the rear bumper cover making it higher than the 2004-2010 units. Anyone seen this in real life?
I have not seen the 2011 in real life - with our without a hitch - but I don't see the bumper cutout in the
brochure, and the detail photo of a hitch in the Accessory section on page 11 shows an installation like the second-generation version (no bumper cover cutout, hitch receiver frame exposed under bumper). Perhaps I'm missing something... which photo shows the cutout?
I notice that the hitch in the photo is not a
DPS design (all Toyota OEM hitches for 2004-2010 were DPS). I also note that the ball mount chosen for the photo looks like a black Anderson Ultra-Hitch Alumistinger, but that brand/model of ball mount (or any aluminum ball mount to my knowledge) is not available in black. Also, I have not found an aluminum ball mount for a 1.25" receiver, and Toyota OEM receivers for second-generation Siennas were 2" units.
Also, that accessory section shows that hitch photo and includes "Towing Accessories" (Canada) or "towing hitch" and "towing ball" (USA), but the online accessory lists (both Canada and USA) do not include any towing equipment at all. Someone has messed up!
The Toyota USA
brochure is different (and much longer). There is a black section along the lower edge of the rear bumper cover which looks like it might be a long thin cutout, especially in the Sport version, but I think it's just a black section of plastic.

Originally Posted by
tcp
WDH capable?
I assume that any 2" receiver hitch would be WD-compatible, as with the second generation. The floor structure is reportedly unchanged from that previous generation, so nearly the same hitches should work and they should have the same ratings and capabilities.