Now, there should be about 1000 different small utilities to convert between wgs-84 and s-jtsk. Apparently they are, but they seem to be written in visual basic and without source. I *know* I have seen C++ sources somewhere on web, but could not find them now :-(.
Oh and btw greetings to Mirek Spousta and Johanka Dolezalova. I did not realize people actually read this :-).