i think it has to do with the abandoned mineshafts, or to be used in conjunction with teh new dungeons Notch is planning.
Edit: I so wish ppl used the default pack when posting screenshots. it makes it much easier to identify things and understand exactly what we are looking at.
Indeed I've only found these in Mineshafts, unlike circle caves ive found those everywhere
EDIT: Sorry I didn't have much time to change textures, because sometimes when trying to reconnect to the server I get an error and I have to restart Minecraft like 5 times, and my friends would have to wait for me. :/