The description of PlanePlotter claims that it can determine the registration letters from the ICAO 24 bit address algorithmically for some countries.
The only thing i got so far is how to get the country. Any clues?
After some more research I found some information in the book "Principles of Avionics" by Albert Helfrick:
Generally, there is no algorithm giving a one-to-one correspondence between ICAO 24 bit addresses and the registration number of the aircraft. Some ICAO member states did have an algorithm that would translate ICAO 24 bit aircraft addresses to registration numbers but ICAO strongly suggested the practice to stop.
Algorithm for swedish registrations: http://www.transportstyrelsen.se/publicdocuments/PDF165.pdf