... is if you were listening to something in stereo and a voice says "I'm over here on the left and now I'm going to run all the way to the right" and he appears to be moving from the right to the left instead. Otherwise it doesn't matter.

Phase is something different though, and that can compromise sound quality.

With a standard symphony orchestra the instruments are usually (but not always) arranged in particular locations. This means that, for example, the string basses are usually on the right of the image, and the first violins on the left.

Likewise there's also set arrangements for the locations of instruments in smaller ensembles, such as string quartets.

Drums are often miked up in a L->R way. So the hi hat may be on left or right and a roll round the kit travels in your head. I like the left to be left (and r to be r) cos that is the way the producer/engineer/drummer intended it. For me, the hi hat should be slightly on the left but I'd be keen to hear a pro's view on this.

on whether you're north or south of the equator.

the right and proper way to do it. If your lady friend (it's always a lady isn't it?) does it again, make her go back into the kitchen and do some cooking and cleaning. Don't let her do any knitting until she promises not to mess with science again.

...someone else to use your earphones? Eurgh.

If you're talking about the buds which come with the iPod, then each bud is shaped for its own ear. If you put them in the wrong way around they keep falling out, because not enough of the bud is actually in the ear.

..hearing the mix as the creators intended.
If that is not important to you..just wear one earphone.

I watch girls sharing listening. One has one earbud, one the other..NO!!

It's like those pubs (Are you listening Mr Duke Of Buckingham in the High Street?) who have their speakers wired in cack-awful fashion so that in the pool-room you're only getting the left channel.

What's worse is that I'm the only person who seems to notice. Bloody phillistines.

I'm no expert in this matter (and a great many others), but I have noticed that some of the in-ear pieces I've had are asymmetric, so the holes through which the sound comes out are off center and tailored to the typical human earole, which presumably means that if they're in the opposite ear the sound will fire into the fleshy bit rather than the ear canal. Dunno how much difference it would make to sound quality you experience though.