Yep, it's for real:
This brand of bottled water was dreamed up by an advertising agency that wanted to gauge how effective outdoor billboard ads could be. So they put 40 billboards advertising the fictitious Outhouse Springs Water up around Charleston, South Carolina. The billboards sported slogans such as "It's #1, not #2!" People definitely noticed the ads and actually started asking for the stuff at retailers. Demand became so intense that the ad agency eventually made a deal with a bottled-water company to produce a limited run of Outhouse Springs Water. It's on sale now at Piggly Wiggly's in Charleston.More here. The Leno gag suggests that the stuff is still out there.
On a somewhat related note, I've heard that there is a unique cheer at Austin Peay State University in Clarksville, TN. "Let's Go Peay!"