Supposedly in the first week, 1.2 million albums were downloaded from Radiohead's official website in the first week, at varying "pay what you want" prices (I've heard the average was $5 - $8, but this is all on non-confirmed secondary site surveys). At the same time, 500,000 people downloaded the album from bittorrent. So even though it was basically nothing to download the album, people often defaulted to the method they were used to getting the music from. Was this to get higher quality files, to avoid the hassle of site registration, or just because why change habit when what you do works - until there's a compelling reason to do so? (And legality isn't a good enough reason for most people.)
Either way it's worth noting that the band only sold six times the number of albums they sold last year - only 300,000. So the buzz alone from the "pay what you want" exercise paid off in multiples.