Dude, this is one guy with a lot of time on his hands.
From what I can figure though, this is just bait to get the clickity-click on through to Mr. Skin which is a subscription(?) service for celebrity nudes.
Here's what he had to say about our lovely Jossverse actresses:
Iyari Limon is not listed.
Laurel Holloman is not listed.
Michelle Trachtenberg is not listed.
Robia LaMorte is not listed.
Sarah Michelle Gellar
Summer Glau is not listed.
I think, according to his entry on Kathy Bates, that his extensive list is compiled not only on his own opinion, but on the search requests for nudes at his other site. Kathy Bates gets a lot of hits. As well she should, dammit. But I'm surprised that Michelle Trachtenberg isn't listed. Maybe it's a legal issue since she portrayed a minor on-screen? The Olsen twins are most likely absent because they turned 18 as of two minutes ago.
ETA: I forgot about Mercedes McNab! Ack! I added her link.