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After implementation, I saw ranks improve, decrease and being the same. The amount of PageRank that dissipates through a 301 is currently identical to the amount of PageRank that dissipates through a link.  http://www.youtube.com/watch?v=Filv4pP-1nw. So a small decay may occur resulting in a different rank. On a second note, what happens many times is that the 301 redirect goes to a page that has more/less content, different template and so on as the original landing page. Since the content is different, the position in the serps may go up/down in organics (this happens even if the page is the same as a competitor's page may become more relevant for a query based on algo changes, content changes and so on). The most successful migrations are the ones that have the majority of the pages 301 to a landing page that is roughly the same (all other things being implemented properly). Key is planning. It's stressful, but feels great when implemented properly. Good luck!