wheel - 11:35 am on Jun 3, 2010 (gmt 0)
About 3-4 months ago I took a ranking domain and did a 301 redirect to a new, prettier domain. I also changed the website content, though some of the pages remained the same.
Rankings took a huge drop. they still have not returned to their pre-301 levels.
Sounds like it's a bad idea to do content change at the same time as a 301 :).