Quote from ig0r:
Fair value should almost never be under 0. It's possible if dividend yields are greater than the discount rate. Also there are arbitrage bounds, so even if it goes negative and fair value is positive, you won't necessarily be able to profit from it due to B/A spreads, funding/lending spreads, etc.
Here's S&P 500 fair value and arbitrage bounds based on this websites estimations of the costs involved:
http://www.indexarb.com/avtceSP500JUN2008.html