Martin Klein et al. have self-archived "Comparing Published Scientific Journal Articles to Their Pre-print Versions."
Here's an excerpt:
Academic publishers claim that they add value to scholarly communications by coordinating reviews and contributing and enhancing text during publication. . . . We have investigated the publishers' value proposition by conducting a comparative study of pre-print papers and their final published counterparts. This comparison had two working assumptions: 1) if the publishers' argument is valid, the text of a pre-print paper should vary measurably from its corresponding final published version, and 2) by applying standard similarity measures, we should be able to detect and quantify such differences. Our analysis revealed that the text contents of the scientific papers generally changed very little from their pre-print to final published versions.