As the word is not usually found in "proper" publications, I was unsure about that and thought that it didn't exist (for proper use), but hell, we say shit all the time and when used as a verb, I compare its usage with the word "spit" which has a past tense "spat" and therefore deduced that "shat" is the past tense of "shit." Well, some thoughtful forummer posted URL links to make it clear the doubts. Anyways, when I use the word shit, it is normally as a noun, or adjective, for the verb, I prefer the Singlish term of pang sai - don't have to worry about past, present, or future tense, it is always pang sai.
Cheers!
NO, it is still SHIT.....