Defining co-authorship

Here is an important article on how co-authorship should (and should not) be determined:

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2544445/

And here is an interesting scoring algorithm on authorship criteria:

http://isites.harvard.edu/fs/docs/icb.topic562342.files/authorship_criteria_Nov02.pdf