How was the magic network ID value chosen?
main.cpp carries this comment:

// The message start string is designed to be unlikely to occur in normal data.
// The characters are rarely used upper ascii, not valid as UTF-8, and produce
// a large 4-byte int at any alignment.
unsigned char pchMessageStart[4] = { 0xf9, 0xbe, 0xb4, 0xd9 };