This week's Economist told the story of falling wholesale milk prices which are bringing EU farmers to call for lower milk production quotas. Some farmers blame supermarkets for the lower prices, noting that they sell milk at about the same price but buy it from farmers at a significantly reduced price.
Falling consumer demand (especially given the recession) isn't the explanation EU farmers give, and it certainly doesn't seem to explain the inconsistency at the supermarket. But the Economist linked them, though they didn't say how. The short version is: it's all about the cheese.
Cheese demand has fallen a lot, especially within the EU. Unlike milk, people buy a lot less cheese when their incomes fall. But cheese is made from milk and in the wholesale market, all cheese is milk (since cheese-makers are buying it to turn into cheese). In other words, grocers who want milk for milk compete with cheese-makers who want milk for cheese. But people want less cheese so there's less demand for wholesale milk.
Just as more cheese-makers bid up the price of milk (and a growing China bids up the price for gas down the street), fewer cheese-makers means there's downward pressure on prices. Grocers, then, are getting their milk on the cheap. But the market for milk-at-the-store changed very little. So we see grocers getting cheap milk, selling it at about the same price, and it's all thanks to consumer demand, not colluding grocers.