Serial.println(F("Hello string in flash memory"));
sprintf_P( buffer, PSTR("Hello number twelve: %d"), 12);
Perhaps this will be fixed some day. For now it is a little inconsistant.
But I've found stranges different results when I use F() and PSTR() in Serial.println. In a sketch that uses ethercard for tcp/ip (high memory usage), I had problems of garbage sent to Serial instead of some constant strings.