Wow, that's really useful.

I've had a similar squeak for about 6 months now.
I'm trying to put off getting my bike serviced!
The squeak always happens when I'm shifting down at the back.
I've spent hours and hours and hours carefully lubing the cables and every single moving part on my little squeaky bike.
I think mine might be the jockey wheel (greased that too, so sounds like it might be worn).
Time for a trip to the LBS I think! Thinking of changing the rear cassette anyway, so will get them to look at it.

Thanks guys!