The entire passage was read for the album by the Macintosh voice program (set to Ralph, I think). The emotionless voice gave the track kind of a creepy, mechanized feel.

Rumor has it this Radiohead song was written by Thom Yorke and an anonymous fan. Essentially, each line was a response to the previous in in e-mail.

The fan wrote the first line, and Yorke responded with the second. And so on. This may be just urban legend, but the idea is pretty cool.

The sample that plays underneath Fitter Happier that repeats "This is the Panic Officer. Section 917 may have been hit" comes from the movie Three Days of the Condor, starring Robert Redford, Faye Dunaway, Max von Sydow, and others. I shouldn't node much more as it's not related to the song, just plays for a few seconds underneath.