Carol Wapshere delivered an excellent session yesterday at TEC 2012 on the thought process for migrating from MIIS/ILM to FIM.
I loved the incisive logic to focus on the main issue being solved: getting the customer onto supported software (getting the MIIS database off SQL 2000, getting off MIIS/ILM). Avoid the temptation to try and fix everything else at the same time. She had a great list of gotchas. Even more impressive were her discovery scripts designed to analyze the existing implementations and her rubric for estimating the work.