Quote from segv:
Hint: To see why the implied volatility skew likely does not represent a trading opportunity, for each strike K, calculate the cash flow from delta-hedging a put over the last X days for an option expiring in X days, then solve for the volatility where the sum of the daily cash flows is zero at expiry. Calculate the slope of the skew and overlay on the strip from the current ATM volatility that expires in X days. Ponder why the slope of the "fair" skew is slightly different than the observed implied volatility skew, then remember certain events in the late 1980s, and further consider transaction costs if they were not included in the delta-hedging simulation.