Some resources I found quite useful so far:
Christian Mensing's already mentioned bandoneon page - especially the section on bandoneon terminology.
The Bandoneon Yahoo Group - everything about the bandoneon.
Tangojam.com - a growing repository of bandoneon resources, especially for those looking for solo arrangements.
Todotango - everything related to tango.
Lightonthestairs - download links to tutors / scores for the bandoneon. (Thanks Tamas Radnai for the pointer!)
Danceforums thread on the bandoneon - this might develop into something useful.
Please don't hesitate to share your resources in the comments!