I first saw the issue in one case, and didn't took care, but the issue appeared again, some more times. so I post here.
When I start an automatic upgrade from WP 3.5 to 3.5.1 (french pack) I always have the error : "impossible de copier le fichier.: /wp-content/upgrade/wordpress-3.tmp/wordpress/wp-content/plugins/akismet/.htaccess" (unable to copy file....)
So, automatic upgrade isn't possible. I've tried to :
* create folders or files with chmod 777
* deactivate akismet before upgrading
but it doesn't work.
maybe a corrupted zip archive ?