Look up WordPress roles by capabilities, instead of having to thread through the role object to check capabilities against every role.

zs_find_role_by_capability

function get_capabilities($cap = false){
  # Get the WP_Roles object.
  global $wp_roles; 
  # Set up array for storage. 
  $role_reversal = array();
  # Walk through the roles object by role and get capabilities. 
  foreach ($wp_roles->roles as $role_slug=>$role_set){
    
    foreach ($role_set['capabilities'] as $capability=>$cap_bool){
    	# Don't store a capability if it is false for the role (though none are).
		if ($cap_bool){
  			$role_reversal[$capability][] = $role_slug;
  		}		
  	}
  }
  # Allow users to get specific capabilities. 
  if (!$cap){
    return $role_reversal;
  } else {
    return $role_reversal[$cap];
  }
}

function get_role_by_capability($cap, $lowest = true, $obj = false){
	# Get set of roles for capability.
	$roles = get_capabilities($cap);
	# We probobly want to get the lowest role with that capability
	if ($lowest){
		$roles = array_reverse($roles);
	}
	
  $the_role = array_shift(array_values($roles));
  if (!$obj){
	return $the_role;
  } else {
    	return get_role($the_role);
  }
	
	
}


# If we want to allow users to set access by role, we need to give
# the users the names of the roles, but WordPress needs a capability.
# This function lets you match the role with the first capability
# that only it can do, the defining capability.
function get_defining_capability_by_role($role_slug){
	$caps = get_capabilities();
	foreach ($caps as $slug=>$cap){
		$low_role = get_role_by_capability($slug);
		# Return the first capability only applicable to that role.
		if ($role_slug == ($low_role))
			return $slug;
	}
	
}