Is there no way for Processing to talk to Mega?
I was using a Mega. So, yes, Processing CAN talk to a Mega.
One thing I forgot to ask was which version of Processing you are using. I am using 1.5.1.
I did the same thing to the Arduino sketch, except that I added:
Serial.print("brightness: [");
Serial.print(brightness);
Serial.println("]");
And, I modified the Processing sketch, adding a serialEvent(Serial port) function, that Processing calls when there is sufficient serial data. I got the correct response in the Processing window, though I did have to read it one character at a time, casting to a character, in order to print it.