Detailed geek-mail is pouring in. Here’s the best so far:

Jonah,
If you recall, back in season two, Spike had one of his minions videotaping Buffy dusting a vamp to study her fighting style. So, “Entropy” was not the first instance of it.
One techie-BTVS fan explains:
“Light entering a video camera lens falls upon a CCD (charge coupled device) which outputs an encoded electrical signal to the tape recording part and
miniature TV view-finder. A vampire probably cannot be seen through the viewfinder of an SLR camera, as a small mirror is used to direct light until the shutter is snapped.”
Thus, the videocamera is not “properly” a mirror or a reflection.
Glad to be of service on such a matter of utmost importance.

And…

Jonah,
Although this is irrelevant geekery, in the Buffy the Vampire Slayer-Angel universe, vampires can be photographed. Note that Angel shows up on camera
footage in the series Angel in several instances (including the series premiere) and that Spike videotapes Buffy fighting vampires in some episode in
the second season.
The logic behind this is that vampires don’t show up in mirrors. However, modern video cameras do not use mirrors. Therefore, the “vampires don’t show up in mirrors” doesn’t necessarily apply. Some extend the “no images in mirrors” rule to assume that they cannot create “artificial” images (this perhaps has to do with the idea of some native types that photographs steal/image a person’s soul, and vampires, of course, have no soul). But there’s no technical reason why this must be so.
Like I said, unimportant geekery.
Brendan

And…

I’m such a geek…
It has been established within the Whedon-verse that you can photograph a
vampire. On an episode of ANGEL, Cordelia and Angel are looking at an old
photo with Angel in it. Cordy remarks, “So vampires DO photograph. The just
don’t photograph WELL.”
God, I’m a geek.
Jeff

And…

Jonah,
I think the reason for Spike’s history-defying ability to be filmed can be explained, circuitously, through one of the Simpsons “Treehouse of Horror” specials. Xena, Warrior Princess (Lucy Lawless) is at a comic store, being grilled by nerds. One of them asks about an apparent flub he saw in one episode. Says Lucy: “Uh, yeah, well whenever you notice something like that.. a wizard did it!” As Buffy creator Joss Whedon has told interviewers he was a fan of Xena, we can assume that show’s logic may be transposed here.
That episode of Buffy is on the new Season Six DVD set, too. Tons of extras – well worth getting.

ME: While I do respect that the Whedonverse is more consistent on this point than I realized, I basically don’t buy the video-but-not-film thesis as far as the science goes (And, as several folks have pointed out, Angel has had his picture taken with film). But believable science is not the lodestar of the genre. Anyway, out of deference to my colleagues I won’t be cluttering the Corner with too much more of this stuff….today.