I've been on the fence about this question for a while now, but flipping through the latest Lenswork magazine last night gave me a little insight, or at least told an interesting story about learning digitally.
There's a portfolio presented from Robert Swiderski of various images of St. Hyacinth's Basilica that are very nice images indeed. After all, they're in Lenswork, whose magazine I admire! Most portfolios can at least be described as "mature".
In any case, after going through some of the images I read the interview and was suprised to learn a couple things. First off, he's shooting with a Digital Rebel which he shares with his wife. Secondly, he's only been shooting for five years with no prior photographic experience outside snapshooting. He has never shot film at all, only digital.
Regardless of how you view his work, this seems to be an impressive accomplishment, especially given that he has a book put together with publishers interested. He attributes his quick accomplishment to shooting thousands of frames of almost anything.
"By previewing images on the LCD it taught me about exposure, how light behaves, and how photography works. It was also something I was doing every day."
With those advantages available, digital certainly has the potential to be a great learning tool, but I doubt that it's fully utilized by the vast majority of shooters. These cameras make it very easy to overlook the learning potential and start snapping, which is of much less value IMO. If one hones in on the meat of photography, being exposure, composition, etc, then it can do wonderful things as we can see here. The allure of digital is the quickness and ease.. someone with that goal in mind would be hard pressed to then slow down and skip the ease of snapshooting.
On a side note, I think at least of equal imortance is lots and lots of viewing of good photographs. Other people's work can be a great teacher to show what quality work looks like so as to have something to strive for. I especially like jlw's example of watching a welder work and actually seeing what a proper weld bead should look like, as opposed to learning by trial and error.
My $.02 🙂