I've always been uncomfortable with the comparison made between lean manufacturing and software development.
If you're mass-producing Toyotas, then you know exactly what steps you need to go through to build the car. Okay, some will be different colours, have different features, etc, but the variation is constrained. The customer gets a set of choices, and has to tick a box. 'Which colour?', 'Which stereo?', etc. Go to your local Toyota dealer and ask for a spec not on the option list and see what he says. The whole idea is to do a relatively small set of predictive tasks as efficiently as possible, anything which does not contribute to the end result is waste.
Software, on the other hand, is a non-predictive, learning activity. If you do something which turns out to be useless in the end software, well that might have been a critical part of how you learnt about the domain and worked out the best solution; it's waste from a manufacturing perspective, but not from a design perspective.
The problem is we're applying lean MANUFACTURING principles to software DESIGN, and the metaphor just doesn't work. However, try applying lean manufacturing principles to what your software does, rather than how you develop it, and it’s a different story.
Let's use retailers as an example. They have to move stock around, and each warehouse/store needs to know what stock is expected in that day, but sometimes unexpected stock just turns up. Someone in the business says that 'some stock movements are just not advised', and we have to cope with it. And since we're IT people and do what the business say, we find a way to cope with it. But what would a lean car factory do in this situation? I think they would stop the production like, work out why stuff is turning up un-advised, and fix that problem, not find a work around to cope with unreliable inputs.
Lean manufacturing gives us insight into how we can structure our predictive, repeatable business processes so they are as efficient as possible, it just happens that if you work in an information industry most of this business processing is done by application servers, not people and robots in factories.