The story behind the connection between Douglas Adams and Pink Floyd's recent album The Division Bell, as he told it himself on alt.fan.douglas-adams:
Dave Gilmour's birthday present to Douglas was an invitation to play one number with the band at Earl's Court in October 1994. That night's show was also in aid of the EIA.
As detailed in the radio scripts book, when Arthur and co first set out onto the barren landscape of Magrathea, a bit of atmospheric music sets in, the intro to Shine on You Crazy Diamond by Pink Floyd (from Wish You Were Here). This kind of music-swiping was standard practice for the first radio series. The joke is that Arthur then remarks "Did you know that robot can hum like Pink Floyd?" When asked what else he can do, Marvin hums Rock and Roll Music by the Beatles. A few moments later, when Zaphod does a dramatic speech about entering into the subterranean depths of Magrathea, Also Sprach Zarathrustra (used in the film /2001/) begins, dramatically, to play under this speech. As the music peaks, Zaphod mutters "Can it, Marvin!" and the music cuts out.
This is one of fans' favorite gags in the first series, but had to be cut from commercial pressings of the radio series on cassette and CD because although Douglas Adams himself has long been on good terms with Pink Floyd and counts most of them as friends, the lawyers didn't know that and rather than suffer under the incomprehensibly torturous Pink Floyd copyright laws, they removed the entire bit. (See Marvin Sings Pink Floyd.) The section is still heard whenever HHG plays on the radio. They just cannot legally sell it.
On the first record album rerecord of the Hitchhiker's Guide to the Galaxy a similar sequence survives, minus the Beatles tune. An atmospheric (if not terribly Pink Floydish) riff plays, Arthur says "Do you know that robot can hum like Pink Floyd?" Zaphod goes into his spiel, Also Sprach Zarathrustra plays and Zaphod says "Can it, Marvin."
You can download an mp3 of the missing bit.
Thanks to Bug for originally writing this answer.