Has anyone noticed that you can often see a faint purple flash when
you open an envelope? This can usually be seen when the gummed areas
are pulled apart, and is easily visible in a dim light. My question
is, what causes it?

The flash you see when opening certain envelopes (especially the
self-seal variety) is caused by triboluminescence.

Triboluminescence is the emission of light resulting from friction
generated by the vigorous crushing or tearing of certain substances.
Sometimes rapidly yanking a piece of Scotch tape off a roll can
produce a small amount of visible light, and the same mechanism causes
the well-known "sparking" effect of wintergreen candy when it is
crushed between the teeth. All these phenomena are much more
noticeable in a darkened room, of course.

Briefly, triboluminescence occurs when molecules are jammed together,
forcing some of the molecules' electrons out of their atomic fields.
The free electrons collide with nitrogen in the air, imparting energy
to the nitrogen molecules, which causes them to vibrate and emit
light. Most of the light emitted is in the ultraviolet range, but they
do emit a small amount of visible light as well, which is violet or
bluish in color since it is in the high end of the visible-light
spectrum.

The reason why some self-seal envelopes do this more than others is
that some adhesives have a greater tendency to absorb the ultraviolet
(short wavelength) light that is produced by the excited nitrogen;
they re-emit this as light in the visible spectrum, specifically
purplish-blue light.

Here you will find an article that poses the same question you've
asked (about the flash of light seen when opening an envelope) and a
follow-up with some interesting references:

Turbo Press
http://turbopress.com/TPNZ:VIEWARTICLE::275
http://turbopress.com/TPNZ:VIEWARTICLE:855948.216:71

Here's an interesting discussion of triboluminescence as it relates to
the "sparking" of wintergreen candy:

The Straight Dope Archives
http://www.straightdope.com/classics/a1_129.html

More candy-sparking, as investigated in a simple scientific
experiment:

Homepage of Dr. Linda M. Sweeting, Towson University
http://www.towson.edu/~sweeting/wg/candywww.htm

Here's an explanation of triboluminescence, with several good links to
other reference materials:

About Chemistry
http://chemistry.about.com/library/weekly/aa060601a.htm

My Google search strategy:

"triboluminescence" 
://www.google.com/search?hl=en&ie=UTF-8&oe=UTF-8&q=triboluminescence

Thanks for asking an interesting question! If anything I've said is
not clear, please request clarification before rating my answer, and I
will gladly offer further assistance.

Best wishes,
pinkfreud

Nice work, Pinkfreud- that'll do for me. 
Usefaul references, too- thanks.

Astounding. This makes me want to rush out and buy some envelopes for
experimentation.

The word "triboluminescence" is definitely going to stick in my mind,
if I can find a big enough slot in which to store it. ;)