I've had freezing problems at about -10C, or 14F with my shimano freehub(deore)... The related problem is that you can ruin your freehub pawls if they don't engage properly. Chip a tooth off and it floats around in the grease causing more trouble. If you're leaving your bike outside in -10C i'd recommend having a shop prepare it for you or ordering stuff from morningstar.
Has anybody found a good way to prepare a freehub without the fancy grease and flushing tool?
Oh yeah, here is a tip.. if you're way out there and she freezes up, urinate onto the cassette to warm up the freehub a little bit!