Log::info() calls Log::write().
There are only two reasons for that method not to write anything:
the log level of your message is below the threshold. For Info to work, the log_threshold in your app/config/config.php file should be set to Fuel::L_INFO or Fuel::L_ALL, as it is the highest log level possible.
the log file can not opened for append.
In all other cases, it should write a log entry.
There was a bug fixed just before 1.0 release that made the Log::info() work even if the threshold was higher, this is why some will get this after updating. To clarify: it was a bug that it worked before ignoring the logging threshold, it should be set lower in config if you want the infos logged.