Krodal:
Perhaps this will be fixed some day. For now it is a little inconsistant.
Ok, I understand.
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. I thought that PSTR was stored on flash, then it can't be modified after uploading the program. Then, I suppose PSTR is copying the string to RAM or something like this. This sould not be problem but I received the garbage... Then I replaced PSTR with F and the problem was solved... but F can only be used in Serial.print and a few places, PSTR works with sprintf, strcmp, etc.