Data Migration Between CMS Repositories?

StyleChief asks: "My employer has decided to begin migrating all of the company's documentation oriented objects and files to a new content management system. The new system seems to have good functionality, robustness, and better usability than our current systems. However, the task of migrating all of the data from 2 or 3 other repositories to the new system seems to be a daunting chore. Automating the process as much as possible is of course my first goal. There are APIs that one can use to do this, but the details quickly become eye opening. Questions of objects versus files, handling their attributes, authorizations, file type identification, shadowing, build integration, versioning, etc., are several of the plethora of issues at hand. Moving perhaps hundreds of thousands of objects from one proprietary repository to another while preserving everything related to that object is the name of the game. I would like to know how others from Slashdot have dealt with similar scenarios. I am particularly interested in the 'lessons learned,' and the problems that you didn't see coming beforehand."