Arduino serial speed

Did you know that the arduino can use arbitary serial speeds? I have connected my UNO to my pc and uploaded the following sketch:

void setup() {
    // put your setup code here, to run once:
    Serial.begin(500000);
    Serial.write("hello");
}
void loop() {
}

This works fine when I connect my pc to the arduino via putty and set the speed to 500,000

I wonder what the maximum speed is. My tests showed that it is 4,000,000. This is 4 megabit/s. About 16 times the memory of an arduino UNO. I am not sure how this could be useful, but it is good to know.