I know you are never supposed to let the positive and negative speaker-wires touch, but what exactly happens if they do?

Nothing. This is assuming the power is not on.

If it is on, you'll either blow a fuse (if you're lucky enough to have one) or short out (meaning = "break") your amp.

Not always.
It depdns on the receiver you have.
Before I bought banana plugs, the back of my receiver was a bit sloppy, and the pos/neg touched sometimes.
The only thing that happend was my receiver would throw up a "PROTECTOR" light, and shut off.
I'm assuming that if my receiver didn't have that, I'd be looking for a new receiver.
Since banana plugs, I have had no problems

It isn't too hard to build short circuit detection into a power amplifier. If you have it, and get a short, it turns the amp off. If you don't, things get hot FAST and something is going to give and pop. A fuse might save the system, but it's still iffy since there's usually only one on the mains input and the filter caps (capable of 100's of amps) are after the fuse.

Okay, well more specifically, I have an Onkyo 575X. Does anyone know what kind of protection it has against this. I'm not planning on trying it, but I just want to know how careful I need to be. Thanks...

Don't ever adjust, remove, plug in, or play with the speaker wires while the power is on. Not too difficult.

I've always unplugged (or turned off the surge protector) the receiver as well... is that necessary, or should just turning off the power be sufficient? But in any case, Stevevt is right... why risk blowing things up when it's easy enough to avoid the problem :>

I've never had a problem simply switching the speakers off (e.g., unselecting Speaker A or Speaker B if the receiver has that option), and not even turning the power off. As long as the signal isn't go thru the speaker cables, you'll be fine.

Thanks for the help guys.