Negative numbers are generally bad, however a recent survey of the 
communications sector in 2011 found that the best performing companies 
actually had negative working capital. While negative working capital 
can be dangerous, especially if liquidity gets tighter, the advantage is
 that you are actually using other companies' money for free. 
http://www3.cfo.com/article/2012/6/cash-flow_cash-flow-operating-working-capital-digitalglobe