Funny you should start a thread about this today....I just (as in minutes ago) finished my online SDI Nitrox course. I've spent a lot of time studying Nitrox, reading, talking with experienced divers, etc....it killed me to think that I was going to have to pay for a Nitrox class and get nothing out of it. I posted a thread on another forum to ask if I was being silly to think that I would not get anything out of a Nitrox class and to see if it would be worth my time and money (turns out, there are a couple classes I would have taken, given the time and money....but it was based on the instructor, the extra things they go over like rescuing a toxing diver, and going further in depth with the information than just telling you how to calculate you MOD -- or even worse, how to program your computer and let it do all the thinking for you). Unfortunately, I didn't have the time or money to do it the way I would have liked to....so I settled for the cheapest class I could find (and that was the online SDI course).
I believe an online course for Nitrox is perfectly acceptable. It should, of course, be linked to a dive shop (with an instructor), as you'd still need to do that practical portion of it....and that person should be able to answer any questions one has. But given some general math skills, the desire to learn a little more, and some self-discipline, I think an online Nitrox course is a good idea.
However, I would have to say that the SDI course is pretty crappy. It suggests that you use a computer to plan your dive, calculate your MOD, have it warn you if you come close to your MOD or oxygen exposure time (without having you put any thought into it....it even says you probably should not dive Nitrox with your computer set to air as you will not get warned when you reach your MOD)....tells you what your EAD is without going through the calculations to show you how they got it....uses a PO2 as 1.6 almost exclusively, only mentioning you might want to be a little more conservative once at the end of the course.
So, yes, I think Nitrox classes can be done effectively as an online course, but I don't think SDI has done a good job of it. If you want a good overview of Nitrox, I'd suggest looking
HERE.
However, a class that requires dives probably would not be a good online course....though maybe an online supplement could be useful.