I also don't believe they were shot in high def (16:9) so there's not much benefit in a blu-ray release.
Seasons 4-7 were shot in widescreen (Region 2 releases are in 16:9), but aired in 4:3. IIRC Seasons One and Two were shot with 16mm film and starting from Season Three, the show was shot with 35mm, so technically the latter seasons should have no problems quality wise on BR.
Blu-Ray release of Buffy/Angel would certainly make me buy a BR player.
And of course while writing this, I just stumbled upon this article
. Oh well.