I also presume that there was no willful infringement. Unfortunately, unlike for copyright, independent reinvention is no defense.
According to the NetworkWorld article in Florian's list of links, the suit applies to kernel 2.4.22.x and beyond. I don't know if that is the first occurrence or just the oldest one included in the suit. Release notes might help narrow in on where this might have been introduced.
I also see that some reports say the suit was initiated in 2009. I need to find where I saw that it began in 2006. I stand corrected. All of the sources say June 2009.