I'm interested in background material on the student dueling clubs of
Heidelberg. German or English sources are fine, online preferred but a
good in-print book is also welcome.

I've read Mark Twain's bit from <a
href="http://www.mtwain.com/A_Tramp_Abroad/3.html">A Tramp Abroad</a>.
While in Heidelberg recently I had some beers at the <a
href="http://www.schnookeloch.de/index.html">Schnookeloch</a>, a small
bar with lots of dueling memorabilia, and what looked to be current
club members drinking at the bar, complete with ornamental scars.

I'm particularly interested in information on the pageantry, symbols,
colours, typography, etc associated with the clubs. That, and anything
about what is remaining of the clubs in present time. For extra credit
I'm curious if there's any connection to Sankt Gallen, Switzerland; I
was just there and saw logos that looked just like the club logos I
saw in Heidelberg.

I will mostly likely blog a small summary of what I learn along with a
link to this Answers thread.

This ought to keep you busy for a while (all the sites at the end).  I
stuck with English sites, searching with:  german student dueling
which gave a great many more.  I haven't read them, of course, so I
don't know how much you will find about politics and attitudes toward
the dueling clubs, or about the distinctions between "Verbindungen"
and "Burschenschaften", etc.
The last site has a nice description of the technique.
Let's, my father-in-law had a Schmiss or two (one marked with
embroidery on his cap), my American sports coach duelled in the mid
fifties in Marburg (studying theology, but it was the cheapest place
to live), a friend of my son's wasted four semesters in too active
participation, being "Fuchsmajor"
http://de.wikipedia.org/wiki/Fuchsmajor
In Heidelberg in the early sixties, students would arrive (proudly?)
late at dances with a fresh bandage aroung their head.
Good thing that you asked about St. Gallen, which found these sites:
http://homepage.swissonline.ch/zod/fuxenschule.htm
http://www.coburger-convent.de/ccblaetter/ccblaetter021/studententag.html
http://www.1881.ch/
http://wo-ist.com/TOP/World/Deutsch/Regional/Europa/Schweiz/Bildung/Studium/Studentenverbindungen/
http://regional.trampelpfad.de/l5n/0/Studentenverbindungen/

So it seems that St.Gallen does have dueling clubs, but there are also
many student clubs in CH and D that do not, but also have beersteins
with their emblems.
CHeck out all the information on the Coburger Convent site, including
the FAQs, which are voluminous.

http://webs.lanset.com/aeolusaero/Articles/Mayday_Cafe--July_05--manly_martial_art_of_the_sword.htm
http://www.fencingonline.com/academy/reginald_magee.htm#DUELLING%20IN%20GERMANY
http://www.fordham.edu/halsall/mod/1900germanstudents.html
http://www.humboldt-foundation.de/de/programme/stip_aus/doc/buka/berichte_99/swartout.pdf
http://www.teutonia-berlin.de/ueberuns/english/english.html
http://www.frontiersjournal.com/issues/vol2/vol2-03_Beck.htm
http://www.swordhistory.com/reviews/interview.html

Any questions?  Myoarin