Also, version 0.29 of my Raspberry Jam Mod supports NBT.
Since minecraft.py doesn't support this (and may not, as this is experimental, and doesn't work in Juice or the PI or mcpiapi), you need to send the command directly via Minecraft.conn. For instance:
mc = Minecraft.create()
mc.conn.send("world.setBlock",0,0,0,63,1,"{id:\"Sign\",Text1:\"Line1\",Text2:\"Line2\",Text3:\"Line3\",Text4:\"Line4\"}")