When I was at the University of Chicago in the early-to-mid 90s, many grad students were more puzzled than anything else by the suggestion that we unionize. (At the time, most of us were complaining about the lack of teaching opportunities, not the exploitation of TAs.) Now, however, Graduate Students United (which, apparently, is not quite a union) is campaigning for better teaching stipends and other benefits. As I've said before, $1500/quarter isn't much more than I earned as a TA in the mid-90s!