I'll answer my own question, now that I have a DVC80 of my own. I hope somebody benefits from this, because aside from the Videomaker article last month, there's very little about the DVC80 published.
As far as I know, mixing more than two sources in a stereo program always requires an external mixer regardless of whether your camcorder has 3.5mm mini-jack or XLR inputs. My little Azden CAM-3 saves the day again (with help from a Hosa 3.5mm stereo male-to-dual XLR male cable). The CAM-3 seems to be made just for this purpose, as Input 1 is panned left, Input 2 is panned right, and Input 3 goes to both left and right -- perfect for mixing a dialog mic with natural-sounding stereo ambient sound.
The DVC80 is a great camcorder, by the way! If you shoot stanard 60i video for profit and are thinking of buying a VX2000, for the same price as Sony's deluxe consumer model, Panasonic sells a true professional unit that you likely won't outgrow. That is, unless you want progressive scan or high-definition video. (And won't need to dish out another $250 when you need XLR audio with phantom power.) If you really need the 2 lux low-light performance instead of 3 lux -- even though you probably can't tell the difference anyway -- then buy the Sony.
The only potential weakness of the DVC80 is its 10x zoom, as opposed to the Sony's 12x. On the other hand, the DVC80's built-in wide-angle lens is equally practical, so it's six of one versus a half-dozen of the other.
As for the new VX2100, I'd have to see its 1 lux performance for myself to decide whether it's worth the extra few hundred dollars. On the whole, I think Sony charges too much for novelty features professionals don't use, such as all the built-in effects and lousy still camera performance. (If I were in the market for a killer all-in-one consumer cam with manual controls, I'd opt for the Canon Optura Xi.) For flexibility, useability, and overall value for shooting standard interlaced video, I don't think you can beat the Panasonic AG-DVC80.