LANDLORDS MOBILIZE!! We called AP Plumbing - the guys we would always call at our old place (who would bill our landlord) - and Tony was able to come by a couple hours later.
After taking off the end cap and letting all the water drain out, Tony discovered a blockage in the pipe. Using an incredibly scary rooter machine, Tony and his guy basically raped our pipe, got out whatever gunk was blocking it and put a new end cap on it. Problem solved...until we have to replace that pipe sometimes in the next month or so to keep it from happening again.
It wasn't a particularly cheap fix, but he did it on a weekend, pretty much right away. We got to tell our tenants that the problem was fixed within only a couple hours. GO LANDLORDS!
However, only hours later, while doing our first load of laundry in our half done kitchen, the washing machine went bananas. Rattled and walked itself out of the nook, and somehow wedged itself between the brand new fridge and the freshly painted wall, while shaking the entire house like an earthquake. Oh man, this will not do. Imagine running downstairs to see a double stacked washer/dryer shaking like a paint mixer on your brand new wood floors. Freaky.
Neal did some research and we think there's some bolt they put on for transportation that wasn't removed during install...if it's not that, this could be a big problem. Stay tuned.