Every since upgrading to WordPress 3.3 Web addresses aren't redirecting because they should. Transformed: domain.com/2010/10/postname/ to: domain.com/postname/

    RewriteEngine On
    RewriteBase /

    RewriteCond %{REQUEST_FILENAME} !-f
    RewriteCond %{REQUEST_FILENAME} !-d
    RewriteRule ^/[0-9]{4}/[0-9]{2}/(.+)$ /$1 [NC,R=301,L]

The final rule won't ever get applied when the previous rule matches. Presuming the http://domain.com/2010/10/postname/ request does not match personal files or directory, the RewriteRule . /index.php [L] will rewrite the URI to /index.php thus it'll never reach your rule. Try moving your rule up to the peak, just beneath RewriteBase /, and duplicate the !-f/!-d conditions, to ensure that it appears such as this:

RewriteBase /

# for 301 redirect    
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^/[0-9]{4}/[0-9]{2}/(.+)$ /$1 [NC,R=301,L]

# the rest of the rules
RewriteRule ^atom.xml$ feed/ [NC,R=301,L]
RewriteRule ^rss.xml$ feed/ [NC,R=301,L]
RewriteRule ^rss2.xml$ feed/ [NC,R=301,L]
RewriteCond %{HTTP_USER_AGENT} !FeedBurner  [NC]
RewriteCond %{HTTP_USER_AGENT} !FeedValidator [NC]
RewriteRule ^feed/?([_0-9a-z-]+)?/?$ http://feeds.feedburner.com/handle [R=302,NC,L]
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]

Also, if this sounds like within an .htaccess file, you have to take away the leading slash within the rule match to ensure that it appears such as this: ^[0-9]{4}/[0-9]{2}/(.+)$

The issue was because of the key slash and never using $3

    RewriteEngine On
    RewriteBase /

    RewriteCond %{REQUEST_FILENAME} !-f
    RewriteCond %{REQUEST_FILENAME} !-d
    RewriteRule ^([0-9]{4})/([0-9]{1,2})/(.+)$ /$3 [NC,R=301,L]

There is a script here will generate .htaccess rules if you wish to change permalinks towards the /%postname%/ structure.

http://yoast.com/change-wordpress-permalink-structure/

My permalinks were exactly like yours, I made use of it to alter them which is working well.