Is there really a difference between different fiber optic cables?

I recently bought the Monster Ultra 1000 and the Ultra 600
for Dolby Digital. Not only am I not able to hear a
difference between the two cables, but I don't hear a
difference between them and a relatively generic optical
cable I had on hand from years back.

No great surprise. The signal is digital, not analogue as it appears you have
realised, hence any effects are minimal.

In answer to your question about the salesman, they're paid to sell stuff not to
be truthful sources of information.

True glass fibre optic cable typically have lower loss than plastic ones, so can
be used over longer distances.

There's also the issue of single-mode vs multimode cables (see graded index et
al) but this only really effects signals with a greater bandwidth than digital
audio AIUI.

That's pretty much what I figured, so thanks for the
confirmation. I do appreciate your input.

You're learning :-)

Cables just need to be competent. The generic one was.

I'm using the PSB Image 5.1 setup, and an NAD surround
preamp and power amp, with LAT International interconnects
and speaker cable (pricey cables, but they were given to me
by a friend who tests equipment for a living). My sources
are a Dish Network 610 receiver and a Sony PS3 for Blu-Ray
movies.
If there are nothing but 0s and 1s going to the preamp, why
would I be able to here a difference between the optical
cables, to begin with? Of course, the salesman insists there
will be a huge difference, but couldn't really explain why.
I've got another 22 days to return them, but would love to
get some opinions nonetheless.
Thanks for any answers you offer. I apologize if this has
been discussed ad nauseum.

Take the digital cable back to the shop for a refund and use the old
one you have.

Never go to that shop again.

Thinking about burning it down, actually. ;)

For short domestic lengths, there is NO difference in sound quality
between digital cables, and even less between optical cables. I have
done several tests myself and seen others done where digital signals
were quite literally passed over a piece of wet string, with no audible
artefacts. I passed a digital audio signal through my own body, and
provided I wet my fingers, the audio got through fine. Digital audio is
*remarkably* rugged, so unless you are sending signals through
kilometres of fibre, the quality of the fibre is irrelevant. Get your
money back whilst you can.

Thanks very much for that. Back to the store...

sounds like snake oil.