Just brewed the Northern Brewer - The Number 8 extract kit this weekend and I am glad that I researched this well before going at it. It says in the directions to put the AIR LOCK on and says nothing about a blow off tube. Luckily I ignored the instructions or I would be mopping my ceiling.
On a side note it is going well, fermenting at about 67 degrees, I will ramp up to low 70's in a couple days. I plan on splitting it in secondary and adding a pound of honey to one batch as well as dumping some New Glarus Wisconsin Red into it. Hope it all works out.