This very question was at the heart of Jeremy Rifkin's The End of Work. Way back in the mid-90s, already, he was noticing that low-skilled work in particular was being mechanized to the point where large numbers of people would be without work.
One major emphasis of his was that work weeks should be reduced in order to give less work to more people. The 35-hour French Work Week was, for Rifkin, a model of fairness, allowing people to have two jobs where needed or to maintain one and have leisure time. In his understanding of labor, the move from the 12 hour day to the 10 hour day to the 8 hour day was progress, in that leisure was a good in and of itself. By adding leisure to more people, overall benefit accrues to society.
Secondly, he forsaw the rise of the non-profit sector to take the place menial, automatable jobs. While I may not agree withe his prediction, the truth is that the "service" economy, of which the non-profit sector tends to be a part, is in fact where the bulk of new labor is growing. The comparative advantage in time of the unskilled lends itself to service, in that non-automatable tasks will always exist, and everyone has the same amount of time.