Sgt_Kickaxe - 10:51 pm on Jun 11, 2010 (gmt 0)
I've just 301'd an entire site to remove a forced /index.php/ in every page url and there has been no loss of search ranking (yet, 1 week later). Google already has all non /index.php/ pages returning ahead of their /index.php/ versions (Bing and Yahoo haven't even started yet).
Overall the traffic is exactly the same this week when compared to last, perhaps a little higher (1-2%).
If there is a loss of rank through a 301 it's either limited to inter-domain moves or a very small loss.