LS1 cranks are hollow through the center. There's a 1" freeze plug (called a "crank oil gallery plug") in the center of the crank (stock crank also, though that one is just an ooch larger than 1", so it takes a $4 GM freeze plug to fit properly ). If the plug isn't installed correctly (apparently mine wasn't - from Eagle, I looked at it after removing the pilot bearing, and it's just ever so slightly tilted to one side) - it will leak.
Or, if you try to use the 'old school' trick of packing behind the pilot bearing (pack with wet toilet paper or grease) to remove the pilot bearing, you'll find out about this freeze plug pretty quick as well. They have been known to fall in to the motor if you do that.
And if the flywheel bolts are drilled all the way through (and not a blind hole), they can leak if you don't put Loctite on them. Though in my situation, it was pretty obvious that the crank plug was the culprit.
Better watch it Travis, or I'll get a plug for your mouth too. Though the City probably won't like me taking a manhole cover for your big mouth.