I just got my Arduino and loaded up the sketch for the first time, so it's possible I may be missing something, but the comments about interval needing to be a long variable are wrong aren't they?
// Variables will change:
int ledState = LOW; // ledState used to set the LED
long previousMillis = 0; // will store last time LED was updated// the follow variables is a long because the time, measured in miliseconds,
// will quickly become a bigger number than can be stored in an int.
long interval = 1000; // interval at which to blink (milliseconds)
the comment would apply to the previous variable (previousMillis)
but as writen interval might as well be a constant no?