I don't really think we'll ever see massive structural unemployment.
I think that people who are really worried about this miss the key point that makes massive automation possible to begin with: it's much much cheaper than people. If the means of production are hilariously cheap, the goods themselves are going to be hilariously cheap. In a wholly automated world, the share of income necessary to ensure a decent standard of living is going to be itty-bitty-tiny.
Other things that keep me from going Luddite:
- Entertainment: Celebrity is a huge part of our entertainment industry, and you pretty much need a person in order to get that kind of cult following. Even if robots can make better music, you can't sleep with them after a show. Likewise, it's kind of hard to obsess over the day to day social lives of disembodied machine spirits. Plus, even if we do hit the singularity, a machine intelligence is going to have a profoundly different experience from your typical meatbag, and I suspect that will show up as differences in our art and culture. Also, just a side note, I have seen absolutely no evidence to discount the belief that human beings would gladly have 100% of their waking hours taken up with various forms of entertainment. Filling that time could make a lot of jobs.
- Self Enhancement: I don't really think people are going to just ignore the possibility of making themselves a lot sweeter with technology. Genetic engineering, biomedical augmentation, and all sorts of undreamed-of technical muckery are likely to show up in the not-so-distant future and make the smartest humans a lot smarter, the strongest humans a lot stronger &c. This mostly speaks against Reid's points in the comments regarding machines getting strictly better than people.
- Scarcity (all gone): We normally call technological progress "productivity gain" for a reason - the average worker can produce quite a lot more. In our hypothetical, we basically have infinite productivity (it takes, on average, zero people to make a product). I think the technical term for this is a post-scarcity economy. Read the references to hear (at length) about what people think of that.
- Redistribution: As Demosthenes pointed out, and Jared Bernstein corroborates the wealthiest few people have taken the majority of recent productivity gains. Right now we're still pretty tolerant of that, but I can imagine some serious Marx-style revolutions if the trend continues deep into double digit unemployment.