Mate of mine is acutally involved with a court case regarding that letter above. He was contacted by iinet and told 20 ip addresses were slected for thousands and that he was basically lucky enough to have his ip address select at random to be used as evidence in a the court case iinet is currently going through against the major studios, Universal, showtime etc..
iinet won the first round but the studio appealed so they are going again. The studios want the courts to reveal the peoples details and names but iinet refuse and will only provide IP addresses, the court so far agrees with iinet.
There were a few shows tagged, Dexter being one of them, so if you downloaded some of the 'tagged shows' you probably got a letter, i'm still waiting for my letter.
I'll keep you posted on home my mate (we'll call him 'Lucky') goes