As a Dvorak typing sysadmin who used to use QWERTY, the vi navigation keys work just fine on Dvorak. As it turns out, once you're used to Dvorak, you see that having the keys next to each other on the keyboard and on the home row doesn't count for anything in any regard, either memorability or efficiency. I had that reservation about Dvorak too, but ... nope. It's just not confusing or a slow down. It's far less of an issue that the sort of kludgy moded editing and reaching for escape. (I'm an Emacs user at heart. I gave both years of usage, and organically ended up using vi for quick edits, emacs for long editing sessions.)
It's sort of like the much-talked-about switch to reverse scrolling in OS X. You may think it's a big deal, but your brain makes the adjustment, and it just doesn't matter. Your vi editing efficiency should be pretty nearly unaffected. It might be very fractionally slower, since the navigation keys aren't on the home row, but .. I never notice it at all. It's definitely not far less efficient.
Not arguing that you should switch, incidentally. I could also touch type 100+ wpm on QWERTY. I never regained my original speed after switching. That's very common for very fast typists. If you don't put systematic effort into speed training, you'll lose some speed for switching. Not that it's that big of a deal.
The first thing I did when upgrading to Lion was to turn off "natural" scrolling. While arguably more natural when using a touchpad (and definitely when using a touchscreen) is entirely unnatural when using a mouse or trackball. It is counter to how every other OS on the planet has worked for 30 years and doesn't provide any efficiency gain.