720x480 is NTSC DV, the format NTSC home camcorders are recording.
It is indeed 3:2.
Why? Because they just removed a few lines which are not visible (because TV's don't show all lines of a picture) to save bandwitdh (and improve the quality, 'cause, you know, DV is compressed)
And if you add the few lines (6, to be exact), you have 720x486, which is close enough (and has 4:3).
BTW: 3:2 = 4:2,7 which is pretty close to 4:3
Oh, the Pixel Aspect Ratio also is important.
The 720x480 and 720x486 has a Pixel Aspect Ratio of 0.900, but LCD displays have a pixel Aspect Ratio of 1.
So, to have the same Frame Size 720x480 has on a television, you'd only need 720x432 pixels...
Complex? Yeah.

But now you might get the idea why all emulators of TV consoles don't look the same on TFT displays - the Pixel Aspect ratio makes a bit of a difference.