DrugMonkey's been on an interesting run on order of authorship (his view is distinctly from his discipline as in Econ and some other fields alphabetical order is the norm)... so this reminds me of a couple incidents at work.
One project I'm on mostly has people from the geosciences or planetary sciences area. Abstracts sent to AGU have everyone on the team's name on them. The presenter is the primary author, but we're all collaborators even if the piece emphasizes something that's not really on our part of the project. The presenter might not be the first author, btw, as each first author can only submit one thing.
Another project just submitted a conference abstract and I didn't get the opportunity to co-author, even though my role has probably been a lot larger and the team is a lot smaller. Also, I wrote about 60% of our project documentation and my name wasn't added as an author (only the person who edited my work and the person who wrote the other 40%). I, of course, raised a fuss and then got my name put on it... but sigh. This is in an area of CS.