I think it's going to take real design effort if we expect people to reuse these devices. I like your idea of the service helping you repurpose gadgets, or maybe even a deposit on your gadget -- send it in when you are done with it to recycle its rare components.
If a digital device only costs a couple of bucks, and a better/faster/cheaper one comes available a year later, the old one is going to get tossed. How many people repurposed their 2005 iPods, and those cost real money?
Culturally we are accustomed to just tossing aside the old, particularly with electronics. We're going to need significant cultural/technical/legal/design shifts to keep up with these changes. And quickly!
I wish I'd grown up with a machine shop, that sounds fantastic. I really like making stuff, but I'm a horrible handyman. I'm much more comfortable making (or just copying) a model on a computer and then printing it out. I think the interesting part of the technology is that it makes what you take for granted accessible to someone like me.
I agree, there are way too many tchotchkes on sites like Shapeways and Thingiverse, and way too few useful products. I wonder what the first "killer app" for 3D printers will be. That said, I think they are already practical for prototyping or DIY tinkering projects.