WP disobeys disabling of the posts revision feature

I disabled the posts revision feature according to this codex by placing this code in the wp-config.php:

define('WP_POST_REVISIONS', false );

But revions are still being saved by the WP. Why?

I am using wp3.1.3 and Twenty Ten theme. I have couple of plugins, and disabling them does not resolve the issue.

wp-config.php is suggested place for such definitions, because at any point later they might get replaced by default values during loading.

See Editing wp-config.php in Codex for thorough instructions.

Also worth noting that it’s commonly said like add to the end, but really definitions should go above this line:

/* That's all, stop editing! Happy blogging. */

Or, if it was removed for some reason, above:

require_once(ABSPATH . 'wp-settings.php');