Hey chook,
From what I've read, standard beer freezes at about -1 degree C. Here's a chart http://1.bp.blogspot.com/-MG0ceaLvdro/UsMGYfii6pI/AAAAAAAAA3o/c2eCbTFmyjo/s1600/EtOH+graph.jpg
As to what temp your beer is in the kegs, at a guess I'd think they'd be at around the temp you set your temp...