Huh. That's interesting. I guess it's akin to learning a second language...? Thank you for posting this -- it's changed the way I feel about learning Dvorak.
I had similar results, and I never even learned to type very fast on Dvorak. I think the key detail is that it forced me to break all of my bad habits I'd developed, and that carried over to qwerty.
After using Dvorak exclusively for two months I can't touch type Qwerty at all. I think you have to use Qwerty occasionally while learning Dvorak if you want to use both (I don't).
