Well, I figured it out myself…
WordPress disables swfupload for if you’re on a Mac and if mod_security is enabled. Don’t want to be rude bit I think this is kinda silly since there’s a small fix available to disable mod_security, and since this is nowhere documented or mentioned in the sticky on image uploading, although the sticky does mentions the .htaccess fix, which means the .htaccess fix won’t solve anything if you’re on a Mac.
If you want to fix it, edit /wp-admin/media.php around line 759:
// If Mac and mod_security, no Flash. :(
$flash = true;
//if ( false !== strpos(strtolower($_SERVER['HTTP_USER_AGENT']), 'mac') && apache_mod_loaded('mod_security') )
//$flash = false;
Comment out the last two lines.