* MEMC shares rally after results
* Dean Foods to spin off unit, shares jump
* BoE cuts Britain's growth forecast
* Futures off: Dow 69 pts, S&P 5.8 pts, Nasdaq 6.5 pts
NEW YORK, Aug 8 (Reuters) - U.S. stocks were set to slip at the open on Wednesday following three days of gains on Wall Street as traders awaited more signals about central bank action in support of a stalling global economy.
The Bank of England sharply cut its forecast for medium-term economic growth in Britain and at the same time gave little indication that it would rush to pour in further stimulus.
Growing expectations the European Central Bank could act soon to contain the euro zone's debt crisis and a Federal Reserve seen ready to take stimulus measures have triggered a recent rally in equities, with the S&P 500 up for a fifth week running and above 1,400, a level not seen since early May.
"We've had a pretty good run," said Lazard Capital Markets' managing director Art Hogan. "Were in a position in the market now where there are no clear catalysts and yet we've been inching higher. The market seems to be finding its path of least resistance."
Spanish benchmark 10-year yields briefly rose above 7 percent, underscoring the cautious tone from investors recently disappointed by lack of coordination from European officials in their efforts to reignite the economy.
S&P 500 futures were down 5.8 points and below fair value, a formula that evaluates pricing by taking into account interest rates, dividends and time to expiration of the contract. Dow Jones industrial average futures lost 69 points and Nasdaq 100 futures were down 6.5 points.
Shares of Dean Foods, which is spinning off a unit, jumped 28.5 percent a day after the U.S. dairy company posted a stronger-than-expected quarterly profit.
Shares of MEMC Electronic Materials Inc rallied 29 percent premarket after the silicon wafer maker reported a surprise quarterly profit on an adjusted basis.
Williams Partners shares dropped 4.1 percent premarket after the company announced the offering of 8.5 million common units.
Futures did not react to data showing U.S. nonfarm productivity rose at a faster-than-expected 1.6 percent annual rate in the second quarter as companies expanded output but only modestly increased the hours worked by their employees.