http://motherjones.com/kevin-drum/2011/09/who-will-tend-mach...
http://en.wikipedia.org/wiki/Luddite
Different jobs will take their place. There are a lot fewer farmers these days, for example. The real trick is to find effective ways for people to get the training.
This is the wrong place to make this argument (it should probably be made at length) but I think the only real solution when we have a super-efficient, highly automated world is to drastically increase taxes to the super-rich and subsidize people with obsolete skillsets. But regardless, we'll no doubt see more interesting developments here in the next ten years. The current unemployment situation is just the beginning, although a lot is going to happen in the meantime.
(This is from a country where about 10% of the population is currently on welfare and we almost de-facto already have the economic subsity system I'm describing)