Definitely a common problem on the Focus.
My '08 is going in tomorrow for the sills to be sprayed under the curve of the rear door (both sides)
My colleagues '07 also has this corrosion in the exact same place.
My boss' '08 also has this corrosion in the exact same place.
I have looked at 5 cars this week (passing in car parks, car dealers etc) that have all had this corrosion, including a '10 plate!!!!
I bought my car in Jan '12, and had the dealer respray the sill's at the end of Jan, by mid-Feb, the paint was peeling off. I had them re-do it after giving them some advice, which they didn't follow as it came back again by summer time. This winter has made them far worse of course, but it is going in to an independent body shop tomorrow to be done properly.
See my photos in my drop box, https://www.dropbox.com/sh/hnjrhlydlfbq0q9/MojtIWuAx0