The flight plan only serves one purpose: to coordinate search and rescue in case you go missing. Knowing when you will cross which border helps coordinating the search mission.
You don't need to talk to anyone on the radio nor announce your intentions, unless you're entering airspace requiring permission.
I had one situation where things went wrong. I was flying from Krakow, Poland to my fuel stop Vilshofen, Germany (very south east) and filed a direct route through the Czech Republic. The weather was terrible and the mountains between the Czech Republic and Germany were obscured so I diverted south and followed the Danube valley through Austria and eventually Germany. My original flight plan did not touch the Austrian FIR so it was not forwarded to Austria. I was on FIS all the time but when I finally arrived in German air space, there was joy all around! From the AIS point of view, I disappeared when I changed course to Austria and they had already initiated the uncertainty phase and started calling places. Not sure I could have done anything to prevent this as I never intended flying through Austria but I learned a bit how AIS works.
Last edited by achimha; 19th Jun 2012 at 16:10.