I don't know how many of the writers of extensions to GPX 1.1 are hanging out here (and if you are one, you really should be plugged into the gpsxml yahoogroup - the s/n ratio is very good and the traffic is low and of good quality, IMO) but you should take note of Dan Foster's post this morning:
http://groups.yahoo.com/group/gpsxml/messages
As GPX 1.2 will contain stricter checking for your extended schema.