And DRUK, if you hadn't been a "customer" (ie paying your money) you wouldn't have had the pleasure of being a passenger! (OED - "Customer - a person who buys goods or services from a shop or business")
True, I'm both. But "passenger" is still the most appropriate term to describe my situation.
And it's the only one that applies to the dead-heading crew member sitting on my left, and the chap on my right travelling on a staff ID100 - neither are "customers", but they both still have to put their seat backs upright when the F/A tells them to.
Symptomatic, sadly, of the gradual dumbing-down of the English language - why bother using a precise term when a woolly one will do?