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