Digital Bits once again weighs in on the Buffy 4:3/16:9 debate, with columnist Jeff Kleist providing some great information:

As a Buffy fan from the beginning, I have always loved the show's unique style, quirkiness and (most importantly in this case) cinematography. Seasons One and Two of Buffy were shot on 16mm, and while Season Three was shot in 35mm, there are no widescreen masters (though 16:9 extractions were done later on certain scenes for flashbacks in Season Four).

Starting with Season Four, as per an industry-wide move toward HDTV, the show's production company, Mutant Enemy, began to prepare widescreen versions of Buffy episodes (alongside the regular 4x3 versions) for foreign and later domestic widescreen broadcast. The typical technique for this is to center all of your shots within the 16x9 frame, and then extract your 4x3 image from the center. Now the big question that pops up here is, which format is intended, and which is being protected for? While shows like 24 are quite obviously intended for 16x9 presentation (just look at all those split-screens), Buffy is not and never has been. The one exception to this hard and fast rule is the musical episode, Once More with Feeling. Compare the cinematography there to other Whedon-directed episodes, and you'll see a gigantic difference in everything from camera placement to the blocking of actors - a trend that continued in the episodes of Firefly that Joss directed (Firefly was shot for 16x9 for every episode).