MLP FiM S2 EP14. Copyright Hasbro. Please check out my new PMV: http://www.youtube.com/watch?v=kWUok2EyZg4 /)^3^(\
This is the first time the animators have referred to the wall-eyed pegasus as 'Derpy Hooves'. The name was thought up on 4chan, and quickly became the unofficial name. Soon after, she was officially referred to as 'Ditzy Doo' - although Lauren Faust admitted this was merely a placeholder name. To recognise the overwhelming support from Bronies and Pegasisters everywhere, they have now made the name, 'Derpy Hooves', canon.
The voice actor for Derpy is Tabitha St. Germain, who also does the voices of Rarity, Granny Smith and Luna.
UPDATE: The Last Roundup has been re-uploaded onto iTunes, with the Derpy scene being changed heavily. Now, her voice doesn't have the adorable accent, and Rainbow Dash doesn't call her "Derpy" anymore.
The issue has been pretty much resolved. Head on over to http://www.savederpy.com/ to see what Amy Keating Rogers, Screenwriter of "The Last Roundup" had to say on the matter.