If for every 4 cashiers replaced by self-checkout, the self-checkout companies only create 1 job, what's the end game? Will we figure it out before millions get aged out of the workforce in their 40s and 50s, too soon to have accumulated a means to retire?

There isn't necessarily value to "having a job" (especially when you talk about all the unskilled jobs that are going away, which are often jobs that people are doing purely as a means to support their life); there's value to being able to live and have the life that you want. If getting everyone they life they want takes less and less work, cool.

But really, people will likely just keep wanting more and better and we won't run out of work to do

Who is going to pay people the same to work less? Indeed, as we need less and less labor, there will be downward pressure on wages, which will make the situation (for the average worker) worse, not better. The economic actors who benefit most from technological progress are those who own the most capital, and their earnings from that capital are taxed at such low amounts that it won't provide money for the government to sustainably pay people to work less either.

Despite the huge advancements in productivity and technological advancement, we still have a lot of pressure to end or cut back on things like unemployment insurance. Despite the number of unemployed people, we still have pressure to increase the retirement age. Even as we are rapidly approaching the point where many industries could still be profitable with 30 hour work weeks, we push more and more jobs into exempt status, making it far, far more likely that you'll work a 50 or 60 hour work week long before you work a 30 hour week.

I'm not advocating purposely slowing technological progress, but we do need to be aware of the ramifications of it and figure out ways to deal with economic conditions where we require significantly less labor than is available. Our current economic and social constructs aren't really setup to deal with this sort of situation.

That, I doubt. The work that remains requires highly-skilled labour. I think there is a fraction of the potential workforce that will never reach a level at which it can work the remaining jobs. I also think that fraction grows as automation increases (human dishwashers? Gone. Bank tellers? Gone. Bus drivers? Not gone yet, but that could change in ten years). Net effect: there is less work, but we cannot distribute the work that remains over the same fraction of the population.

The trouble, of course, is that we've already been on the automation train long enough that it's not quite so simple as just removing the law now. Is not like flood of low-paying (but greater than zero-paying) jobs will materialize out of thin air.

Capital costs will rise when we run out of cheap energy, but I doubt that will be sufficient.

IMO, the ethical way to get there is through a basic income guarantee (http://en.wikipedia.org/wiki/Basic_income_guarantee).

With it, minimum wages could be abolished. Then that flood of low-paying jobs will materialize, not out of thin air, but out of China. It will, likely, also bring back jobs in shoe repair and the like.

This is probably not a choice i'd make if I didn't live in a city and commute by bus and bike.