unitsapi conversion hook

Nice work, cleaner implementation!

I added a patch, so you could add more arguments to the unitsapi_convert function, those arguments go with the drupal_alter function. (func_get_args()) This way you are extremely flexible.

and if the factor is an array then output them like you do with the temperatures.

I also think you need to test if factor of from and to is numeric, it is possible that someone puts something different in there, like temperature.

else you can get an illegal operand error.

switch ($units[$from]['kind']) {
    case 'temperature':
        $result = _unitsapi_convert_temperature($value, $units[$to]['factor'][$from]);
      break;
    default:
    	if(is_numeric($units[$from]['factor']) && is_numeric($units[$to]['factor'])) {
    		$from_si = $units[$from]['factor'];
		$to_si = $units[$to]['factor'];
		$from_convert = $value * $from_si;
		$result = $from_convert / $to_si;
    	}
    	else {
    		$result = NULL;
    	}
  }

If those factors are not numeric you may assume they are custom, and correctly implemented in alter result;

You can test this if you remove the switch statement and implement the temperature conversion using the unitsapi_unitsapi_result_alter() function :)