Here is a paper that appeared in this month's Physical Review Letters with more than 450 authors (i have not counted this number myself, but guessed it based on the number of lines, and an average number of names per line).
V.M.Abazov et al., Phys. Rev. Lett. 94, 091802 (2005)
The complete list of authors is below
I thought it would not be fair to just copy and paste it, would not have given a true idea of this monstrosity. but it seems rather ridiculous. i need to acquire this paper and find out what its about, sounds more like the entire authore list for the human genome or something like that.