I've been having a similar dilemma about commuting this winter. I've been toying with the idea of getting a singlespeed MTB and converting it to internally geared. More than likely I'll end up keeping it singlespeed just based on cost.
One of these days I need to swing by Bikes Not Bombs to see what they have in stock for beaters. I haven't had a ton of luck with Craig's List, but that's mainly due to me not knowing exactly what to search for.