Comment 1 for bug 16212

http://www.ubuntuforums.org/showthread.php?p=155732 covers this problem, too.
For me it seems entirely random whether you can rename or not, but I haven't
really conducted any scientific tests to find a pattern. It seems to happen
most often on my large (350GB+) reiserfs partition, but that could simply be
because I don't rename things in other partitions as often.