Ditzy Doo IS Derpy Hooves. The lines for that episode were recorded before Derpy was noticed by /co/ and /b/ (It was done all out of order) and the character always was supposed to be ditsy, but the eyes were originally an animation error.
After the Sendai Earthquake, Lauren Faust sold some art related to the show to raise money for charity. The sketch of Ditzy Doo was titled, "Derpy Hooves", officially naming the character. She also seems to be either a mail carrier or a professional mover, as the fans speculated. It is believed a gray filly with a yellow mane and eyes (an Earth Pony) is Derpy Hooves' daughter, though this is unconfirmed.
It should be noted several gray ponies with blonde manes show up in the series, but Derpy is the most frequent. She is rumored to have more lines in Season 2. (I mean we can only go for so long on "Muffiiiiin!") DJ P0N-3 may or may not make another appearance.