Great. My car JUST SITS THERE, due to the p0171 and not having anytime to work anymore on it, and I go out to it...and see a half inch of water pooled in the floorboard, rear passenger side. :/

I'd check the rear window seal

And the door seals.

Drivers side door seal, look at the right side where it starts to slope down. The seal will pull away at this point.

Passenger side, look at the left side where it slopes down.

EDIT:
Do you have a sunroof?

CHeck the floor board on your front passenger side as well. Could be one of your drain hoses has come loose.

If you are within driving distance to 06082, I'll fix your LB1 for you.

What is this drain hose you speak of? I found the exact same thing this weekend on a trip to MD. I thought that it was water finally thawing out from this deep freeze we've been experiencing in NW PA.

Well atlest in the 00 plus models the sunroof drains run all the way down to the floor boards. GM did a **** poor job of connecting them, so they are prone to detatch and spill all of that lovely rain water into your car under the carpet and padding.

no sunroof. How do I check the door seals?
And will I have to get new carpet/under padding?

Are you sure it'* water and not ANTIFREEZE?This happened to me several years ago and it turned out to be a leaking heater core.The coolant ran down under the carpet and collected in my rear passenger footwell.