Solo diving is a question that gets a great deal of debate and requires careful consideration. Each diver is different and has a different definition for what represents risky behavior. As someone who has a wife and 2 little kids, solo diving is something I will never do. This doesn't mean I think no one should ever attempt solo diving -- it's just my decision based on my own risk tolerance. I am also a relative newbie (25 dives) and have no business solo diving from an experience standpoint.
For someone who has 100+ dives under their belt, dives a few times every week (skills are VERY current) and finds that they are self-sufficient in and underwater (good physical and mental shape, redundant equipment, etc.) -- solo diving is probably worth considering. Dive conditions are also an important consideration -- solo dives should probably be dives where you are not pushing the envelope (sites that you already know well, shallower depths, low/no danger of boat traffic, no currents, etc.). One other factoid -- TDI/SDI is now offering a solo dive certification. I don't want to open the "solo dive cert" can of worms here, but one of the requirements they are putting into place is that participants must have 100+ logged dives to participate in this course.
Hope this is helpful.