> I've thought about a Lego-building robot made of Lego before, but mostly in the context of an illustration of why atomic assembly by nanobots is probably impossible.
Bacteria are proof that nano-assembly is possible, if you define assembly broadly enough.
Sure, it's most definitely possible with a limited palette. Anything you can build out of proteins is easy. Other stuff is either harder or impossible, which is why I believe that nanotechnology will enable all sorts of great things, but a "universal assembler" will not be one of them.
Bacteria are proof that nano-assembly is possible, if you define assembly broadly enough.