I believe this thank you was for the redesign of the zombie pigman skin to have green 'blood' rather than red blood, which was what Notch originally had on the skin, however Notch did not want Minecraft to have blood in it. So when Xapholia (or whatever the name is) edited the skin to have green blood, Notch used the texture and thanked Xapholia.
EDIT:
The green liquid in the Zombie Pigman skin was originally blood red, but this was changed because players expressed a preference for the lack of conventional blood and gore in Minecraft. The skin file has "thx xaphobia" written on it as a credit by Notch to the skin's original creator, XaPhobia, also known as X__x
Creepers. That is all.
EDIT: -Wiki
It's XaPhobia.