There could very well be a logical connection between Arduino and Mindstorms.
I've recently looked to see if anyone has done anything with Mindstorms and the Arduino; I noticed at least one person who created these little pieces of acrylic that allowed you to mount the Arduino (and other non-standard things, like servos) to Lego.
The thing that Mindstorms didn't have (well, perhaps before the NXT series) was more than three inputs/outputs on the controller. Short of various modifications or special adapter electronics, there wasn't much you could do to get around that limit. With NXT, they use an I2C bus (or something similar), as well as more easily sourced standard connectors (I think RJ-11?) to the motors and sensors, plus the controller is a lot more powerful overall.
This, of course, comes at a price; I invested a bit of money into my RIS, and didn't do much with it. After thinking about it, I realized that for prototyping small table robots, some larger carpet rovers and the like, Lego was a pretty good platform. But there were drawbacks.
Mainly its cost; for similar amounts of money I could build a stronger and larger robot using standard materials and components. While it certainly wouldn't be easier (or cheaper) to prototype things this way, when what you want is a finished platform you won't take apart later, Lego just wasn't cost effective in the long run. If you are the kind of person constantly trying out some new robotic idea, though, Lego works great for that (but at a certain point, you have to ask whether the money would be better spent on Fischertechnik).
Something that I am keeping an eye on is the Contraptor project (http://www.contraptor.org/); standardized, open-source CNC hardware. While seemingly geared to CNC, such a standard can easily be applied to all sorts of construction (see http://www.gridbeamers.com/) - so other kinds of machines like robots and such become possible.
I envision that someone out there is working on a completely open-source PCB assembly line, with Contraptor-like pick-n-place machines assembling SMT PCBs that were just routed on a Contraptor CNC system; I certainly don't see why it wouldn't be possible to build such a system in one's garage (you could even do IR/hot-air soldering of the components, potentially; wave soldering might also be possible - heck, people make homebrew laser-cutters with custom CO2 laser heads, so why not?).
