This morning, I'm having breakfast and watching a little FiM, and I start the episode The Last Roundup. After the opening credits, there's a roughly one minute set piece involving Derpy.
Derpy talks. And she's called "Derpy" by Rainbow Dash. And the scene is funnier than hell. People at other tables were looking at me.
Derpy is voiced by Tabitha St. Germaine. She also provides the voice of Rarity.
However, I'm about to endorse Internet piracy here. The original cut of the episode (the one I have) has Rainbow Dash calling her Derpy. However, if you go on iTunes, the original version of the episode is pulled. Instead, there is a "remastered" version where Dash not only doesn't call her by name, but Derpy's voice is changed. The episode will be appearing on a DVD, but as I haven't found the DVD and acquired it yet, I can't tell you which version is on the disc.
So what I'm saying is, Fuck the Orwellian revision. She's Derpy. Get the version that proves it.