Negative volumes returned from MtGox websocket api request
So I ran a test where I placed an order that I knew would be unique and then promptly canceled it, the whole time logging all depth objects to my terminal 

The result was 

{ price: '66',
  type: 2,
  type_str: 'bid',
  volume: '6.66',
  price_int: '6600000',
  volume_int: '666000000',
  item: 'BTC',
  currency: 'USD',
  now: '1370548662105838',
  total_volume_int: '37301280846' }


and 

{ price: '66',
  type: 2,
  type_str: 'bid',
  volume: '-6.66',
  price_int: '6600000',
  volume_int: '-666000000',
  item: 'BTC',
  currency: 'USD',
  now: '1370548667379328',
  total_volume_int: '36635280846' }


Which leads me to conclude that negative volumes are in fact the result of a canceled trade