An Experiment in Hedging With ETFs - WSJ.com:
"ProShares warns investors that it aims to have the funds' movements correspond to the inverse of the relevant market indicator on a daily basis. Over more time, the effects of compounding can (and generally will) dilute or enhance the results. For the second quarter, when the S&P 500 gained 15.93%, the Short S&P500 lost 16.03%. Year-to-date the total return for the S&P 500 is up 20.10%, but the short fund is down 22.63%."
and again, it is much worse for leveraged funds.