For the Halloween '11 bunnies, I call them Bat Bunny, Witch Bunny (the striped socks make me think of the Wicked Witch of the West), Halloween Nick Bunny (since Nick's name is spelled out in the markings), and Lichen Bunny.
In the discussion thread for the release, people seemed to be calling the blue one 'witch/blue/Frankenstein' (I don't really agree with the Frankenstein name myself since the hair is long and it seems to be wearing a dress), the bat bunny just 'white,' the one with Nick on it 'Nick bunny,' and the last one 'green,' though it was also mentioned that the green one looks kind of like it had pumpkin vines on.
search.php?st=0&sk=t&sd=d&sr=posts&keywords=bunny&t=843331&sf=msgonly&ch=-1Edit:
For this '11 Easter Bunny:

the artist said in
this post that it was inspired by doves, so Dove Bunny is a possible name.
This one (also Easter '11):

I've taken to calling Inverted Dark-Faced PPS. It was one of the free bunnies, so it's not
actually based on an existing pet, but if you took the Dark-Faced PPS from the Jungle Litter (Oct '08) and flipped the dark and light areas, it'd come close.