Since I'm just re-reading OotP now, I didn't remember the bit about Dumbledore admitting he sent the Howler to Petunia. And as for why Harry didn't recognize the voice, I imagine you can disguise your voice pretty easily when "recording" a Howler.

I'm at Veritaserum.com now-- where's this article?

Interesting thing I just discovered-- the terrorists that attacked those buses in London supposedly congregated at King's Cross station... HMM! (Buncha Muggle-haters!)

And by any chance do you know why I can't change my icon when posting comments? /.\