Conqueror

It’s trivial to add this to /wp-content/themes/twentyeleven/functions.php

function is_multi_author() {
  global $wpdb;
  $rows = (array) $wpdb->get_col("SELECT DISTINCT post_author FROM $wpdb->posts WHERE post_type = 'post' AND post_status = 'publish' LIMIT 2");
  $is_multi_author = 1 < count( $rows ) ? 1 : 0;
  return (bool) $is_multi_author;
}

I’ve found that wp generates incorrect nonce in some conditions. Reason: see above

Trace function “check_admin_referer()”
and especially lines

$result = isset($_REQUEST[$query_arg]) ? wp_verify_nonce($_REQUEST[$query_arg], $action) : false;
if ( !$result && !(-1 == $action && strpos($referer, $adminurl) === 0) )
{
  wp_nonce_ays($action);
  die();
}

It’s not bug but rather configuration issue. WP-DB-Backup works fine on a fresh installation.

Unique key defined in wp-config.php (AUTH_KEY) must be identical to $wp_default_secret_key in default-constants.php

Hi Alice.
I had the same problem after upgrading to 3.0.1 and i finally fixed it!
Seems Opera 10.10 has a bug (fixed in modern versions) that means createContextualFragment() (non-standard Mozilla’s extension to the Range object) is unreliable sometimes. Huh, yet another incompatibility issue between Opera and Tiny MCE. But i patched tiny_mce.js and now it works well. Download unpacked script from SourceForge and replace string
r.insertNode(t.getRng().createContextualFragment(h));
with

/* Opera-specific bug workaround: createContextualFragment() loses child nodes */
if (opera && opera.version() == '10.10')
{
	var wa_frag = t.getRng().createContextualFragment(h);
	var wa_stor = document.createElement('div');
	wa_stor.innerHTML = h;
	for (var curr, i=0; curr = wa_stor.childNodes[i]; i++)
		wa_frag.appendChild(curr.cloneNode(true));
	r.insertNode(wa_frag);
	opera.postError('Range.createContextualFragment() fix applied. See tiny_mce.js for details');
}
else r.insertNode(t.getRng().createContextualFragment(h));

If SCRIPT_DEBUG is not defined, then wp-tinymce.js.gz also need to be patched in a similar way.