Someone (willgorman on YouTube) has built a nice functional LEGO version of a rapid prototype machine. MakerLegoBot uses standard LEGO bricks to build small models one brick at a time. Inspired by RepRap & MakerBot, this is a fairly impressive implementation using 3 NXT's (yet still looking slim and svelte!). I don't even want to embed the YouTube video here, because a creation like this really needs a nice written description - which they provide over at BattleBricks. Take a look... and if you are going to Zwolle, this should be there as well!
I wonder how far you could take this. A long time ago (OK, 2005), Ian Hendry of rtlToronto built an automated duck assembly robot that was really amazing. It only built one thing (LEGO ducks... but nice cute ones), but did it from raw pieces, not pre-built sub assemblies, and did it with a single RCX. More recently there was the automated car assembly robot, but it was huge and worked with pre-built assemblies. And there have been some similar creations over in Europe, involving tables-worth of LEGO and days to set up.
But here, it's reachable with a much more modest investment in hardware, and at the same time is much more flexible - given it a digital build file, and it gives you a real LEGO brick-built model. Fantastic implementation... and perhaps opening up some new doors for future variations? One can hope.