Thanks Dantodd for the explanation. If you would like a little more information on diacetyl, you might go to http://brewingtechniques.com/library/backissue1.2/fix.html.
The temperature will have an effect on the rate of metabolism of the yeast and thus the rate of diacetyl formation, but fermenting at cooler temperatures will not inhibit the formation of diacetyl. Lager yeast will still make plenty of diacetyl at their optimum fermentation temperatures (50 -55 F). So, even if you start your fermentation at say 50 F, the fermenting beer will have high diacetyl levels during primary fermentation. Once the yeast have utilized most of the mono, di, and triose sugars, they will begin metabolizing the diacetyl and thus reducing the levels. Breweries do a diacetyl rest (warm the fermentation for a short time) to speed the assimilation of the diacetyl by the yeast so they can package and sell the beer sooner. If you are not in a hurry, you can let the beer sit at fermentation temperatures (50 -55 F) longer and the yeast will continue to lower the diacetyl levels below flavor threshold. Because diacetyl will be made in abundance anyway during primary fermentation, I am not convinced DiegoPro needs to go to great lengths to cool his wort to 55 F initially because of diacetyl. If he gets it to say 70 F, pitches his yeast and then places it at 55 F, he should not have a problem with diacetyl if he either ferments for sufficinet time or does a diacetyl rest.