I brewed a milk stout (Left Hand clone) a month ago and it developed a minor lacto infection. At first I was concerned that maybe my lambic solera is bleeding lacto, pedio and brett into the house and it got in the air or seeped through the plastic of the bucket fermenter the milk stout was in. Then I realized that was highly unlikely to be the case since the milk stout has been on a different floor of the house and was thoroughly sanitized prior to filling. I have since tasted it and it is wonderful without any noticeable lacto qualities.
Then I made the mistake of pitching a smoked porter onto the cake from the milk stout. That definitely got a worse infection. Rather than the floating white chips it got a full on pellicle. It was kind of gray. Really weird looking. I bottled the smoked porter tonight and the taste I had of it didn't taste very sour. I guess we'll see how it goes in the bottle.
I've noticed from pellicle and infection threads on homebrewtalk.com that there are often lacto infections in dark beers but less so on lighter beers. I'm not sure if part of the cause is that more lighter beers tend to be either higher in alcohol or higher in IBUs, both of which will inhibit lacto growth. My belief is that dark beers tend to have unfermentable sugars and/or some other quality that permits lacto to grow even though the IBUs are too high and the alcohol content should inhibit growth. I've yet to read anything in my research that suggests why lacto enjoys the dark beers. My only hope is that I have sufficiently sanitized my equipment to kill off the infections.
It's probably a good time to spray down my fermentation area with lysol just to be sure I haven't created a breeding ground for lacto infections.