I also have this problem, and using
define('WP_POST_REVISIONS', false );
definitely does NOT work - revisions are still being generated. (as earlier poster noted, I also tried 0 instead of false, still didn't work)
I also do NOT want to use a plugin to do something that should be able to be accomplished simply using this wp-config.php file - this define statement above SHOULD work but is not.....so it does look like a 2.8.1 bug - any other suggestions (and please don't say a plugin, I don't like to use plugins to fix a bug or do minor tasks that can easily be handled without a plugin).
This really should be an option under the Settings - allowing us to turn off this feature or if left on, choose how many revisions to keep - 0 to all......