According to a piece (in French) in the French site vitisphere.com, research in cellars in Burgundy, the Loire and the Rhône has found brett in 50% of pre-bottled wines; other studies have found even greater prevalance. The picture is further complicated by several factors: there is more than one strain of brett; brett can be stable or can grow and render a wine increasingly undrinkable; tasters vary in their ability to notice its presence; and finally that some professionals believe that a little brett can add welcome complexity. (This last notion which will strike many New World winemakers as heresy, would incidentally make sense to brewers of traditional Belgian ales).
In any case, for those winemakers who'd rather be rid of this hitherto untreatable ailment, there may be some good news on the horizon. Researchers at the Institut Francais de la Vigne et du Vin (IFV) in Bordeaux have found that electrical shocks may finally be the way to rid the industry of this nightmare. Between two and 50 brief, intense shocks of 1-10 micro-seconds are applied, using highly sophisticated equipment specially developed for the experiment by the French firm Thomson. It seems that the shocks work by rupturing the brettanomyces cell walls.