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I thought it was passive, too. I've googled a little in the past and some people say it's in the same chip as the cellular radios (and the whole chip gets knocked out in Airplane Mode). Others say the amplification of signals necessary could cause interference.

A radio receiver often uses a signal of an intermediate frequency: http://en.wikipedia.org/wiki/Intermediate_frequency

So a receiver that would appear functionally to be passive could still conceivably cause interference.

Cool. Was not aware of the usage of intermediate frequencies in radio receivers. Thanks for the knowledge :)

Phones often use A-GPS, which includes an active component: http://en.m.wikipedia.org/wiki/Assisted_GPS

This is true, but it seems like it should not be hard to disable AGPS in airplane mode.