The plusses and pitfalls of physics co-authorship
Apart from the usual benefits (and possible negatives) of co-authoring a paper, the world of high-energy physics (HEP) has an unusual situtation where hundreds of people can be listed as co-authors. See this paper for an example of 'hyperauthorship'. This arises due to the unique nature of the large experimental apparatus which the physicists work on - much too large to be funded by a single insitution. Thus, large international groups form around these facilities to work on specific experiments (such as CMS and ATLAS at CERN).
So far so good, but HEP has a history of extremely inclusive author lists - so all members of a collaboration are often listed on any paper authored by any member of that group (so long as it relates to their particular experiments).
This, naturally, leads one to question what it means to be a co-author of a HEP paper. How much work has gone into a specific article by each member of the group? Could one outstanding member be responsible for building up the reputation of inactive or less-gifted colleagues? Given that many academic positions in physics, as in other disciplines, relies on the frequency and authority of published research - it is no surprise that this matter has attracted the attention of other researchers.
For a much more comprehensive and indepth investigation into this phenomenon, see Jeremy Birnholtz's paper 'What does it mean to be an author: The intersection of credit, contribution and collaboration in science'. I believe this was recently published by the Journal of the American Society for Information Science and Technolgy, but a preprint version is available from Jeremy's site.