I realize I am kind of late on this topic, but here are a few rambling
thoughts on the subject.  (I consider myself near-expert when it comes to
canker sores, a malady that has plagued me -- as someone else said earlier,
maybe the term "demonized" would be more appropriate -- for as long as I
can remember).

First, let's get terms out of the way.  Canker sores or mouth ulcers are
the roundish depressions found on the soft tissues inside the mouth (just
about everywhere except for the roof of your mouth and, of course, on the
teeth) and are caused by a combination of diet and stress  -- gluten
intolerance is probably right on.   Although sometimes people call canker
sores cold sores, I'm just talking about the sores that appear inside the
mouth.  Cold sores, on the other hand, are found outside the mouth and are
caused by the herpes simplex virus.  Not sure what brings them on since
I've never had one.

For years, I've had maybe two canker sores a month.  Sometimes more
sometimes less.  Generally, they take about a week to go away and, although
you can minimize how bad they'll be by taking care of them, there's nothing
you can do to make them go away more quickly.  As many can attest, they
drive you nuts but aren't going to kill you or anything.

Since going paleo three or four years ago, I've had exactly three total
(compared to two a month pre-paleo).  And all three times were a direct
result of cheating.  To say that my quality of life has skyrocketed would
be an enormous understatement.

Now, I made a post on here a few months ago about a different oral problem
that I've had twice since going paleo that far exceeded the worst bout with
canker sores by tenfold at least.  What happened (sorry to those who have
heard this already) was this.  Went on vacation two years ago in the
mountains of New Mexico with a group of folks who supplied the food.  Miles
from civilization, I was "forced" to eat bread, corn chips, and other paleo
no no's.  I ended each day both sunburned, dehydrated and totally
exhausted.  I used tobacco both by smoking cigars and chewing.  What
happened next was unbelievable.  I developed sores inside my mouth like I
had never seen before.  They were white and patchy and would bleed when you
touched them.  And they were EVERYWHERE, covering not just the inside of my
mouth but way back in my throat as well.  My throat closed from the
swelling and early one Sunday morning I drove myself to the emergency room.
They had no idea what it was or what to do, but gave me some lidocaine for
the unbelievable pain.  My doctor too had no clue what it was or what
caused it.

Second time it happened was last summer (you'd think I would have learned),
same circumstances.  Fishing trip.  Dehydration.  Too much sun.  Non-paleo
food.  Tobacco.  Stress.

This year, same trip.  Same factors but one.  I stuck with the paleo.  No
oral problem.

What was it?  Oral candida or thrush.  Another way of putting it would be
to call it a yeast infection of the mouth.  My doctor did not EVEN suspect
thrush, since it's usually reserved for the very weak (such as extremely
elderly folks) or immune-deficient (such as AIDS patients).  But there it
was.  I owe thanks to Kathleen for helping me figure it out.