Rec709 and sRGB use the same primary colors. Rec709 is the specification for digital HDTV; sRGB is the corresponding spec for still pictures, used almost universally for pictures posted to the web, default digital still camera output, etc. The jpg (JPEG) file format generally assumes sRGB color space.

Other wider gamut color spaces are available for working with still images, but are likely to cause errors unless used in applications that are properly color managed, such as Adobe Photoshop and certain web browsers, when used with files that support color profiles. Therefore, I can look at true wide gamut photos that I take myself on my wide-gamut HP monitor, but I can't post them to the web for anyone else to see.

There have been proposals by Sony and others to expand the color space for HDTV. Expansion has been included in the Blu-Ray specs, but as far as I know, no discs have been produced using it. It has not been adopted into broadcast TV standards.

Is ATSC gamut limited in the encoder or could an expanded gamut be transmitted by "errant" program material that might be, say, conforming to a proposed WCG standard?

The limitation is partly in the encoder, and partly in the camera. Cameras are matrixed to match rec709 primaries. If the camera RGB values are not allowed to go negative or exceed 100%, then the gamut is limited in the RGB signals. If the RGB values are allowed to go negative, (and therefore the color difference signals go outside usual values) before coding, this is one way to carry expanded gamut. This is the basic idea of the xvYCC coding in the Blu-Ray standard.
[Edit:Hypothetically this could also be done in an ATSC signal.]

http://en.wikipedia.org/wiki/XvYCC.

The idea is that non-xvYCC aware receivers simply overdrive the positive R, G, or B and clip the negative R, G, or B, which produces some distortion, similar to that with NTSC sets using non-standard phosphors in the past. That is, colors outside the Rec709 gamut are reproduced with some excess brightness in Rec709 receivers.

Another way to handle different gamuts is to use metadata and profiles to send the expanded gamut with the same range of R, G, B values (zero to 100%), which is the way it's done for color-aware computer graphics programs; but existing Rec709 receivers don't do anything with profiles and therefore would produce desaturated color. This effect can also be seen on computer programs that are not color aware, for example, by coding a jpeg file as wide gamut and then trying to view it on Internet Explorer.

Samsung LED-DLP not the only one offering WCG,
Now Sony Triluminos expands Color experience beyond restrictive ATSC/Blu-Ray standards

Sony has announced that they will use expanded gamut on (some?) 4k Blu-Ray discs.
The claim that expanded color gamut gives better flesh tones is pure hogwash. Flesh tones are well inside the gamut of all devices. In fact, the one-sided expansion of gamut (in the receiver, but not in the source) has to be done carefully to avoid messing up flesh tones, while other colors that you can't be sure of are expanded.

Premium Receiver models have to do what they can in the face of recalcitrant
industry leaders with standards stuck in the 1950s.
Even ice & snow looks way better in WCG, not sure why.