Not wanting to nitpick, but DVDs are the actual physical discs the movie comes on. iTunes don't do those, and probably never will. Are you referring to iTunes Extras?
I THINK what he meant was to actually buy the movie with all the options and extras that are offered on a DVD, instead of just the actual movie, which you could then burn onto a DVD on your own.
But I could be totally off.
Yeah, that's what iTunes Extras is.
So the answer to the original question is, "You already can," unless it's whether there should be more
available, in which case Apple's working on it.
Since Apple publicly released the code it uses for iTunes Extras, I've been dabbling in converting some of my physical DVDs to that format. Takes a long time, especially if you're trying to copy the menus exactly. But iTunes Extras have different menus to the DVD versions, so that makes it easier for them.