Interesting thread, as I just posted a blog entry about GPS interval in Mapper:
http://blog.mardy.it/2010/04/mapper-...tery-life.html

Although I was mainly thinking about having a longer interval to reduce battery consumption, it can indeed have some beneficial effects on the GPX file. I guess that for walking a 30 seconds interval might be the best, but for running or cycling I would opt on 10 seconds, while I'd keep 1 second for car navigation.