Matt Biddulph writes about connecting a physical sensor to an object in Second Life (there's a neat movie). The connection is one-way (from sensor to SL object), although presumably it could work the other way too (SL object to sensor).
I'm interested in exploring the use of Second Life to mirror the real world (David Gelertner wrote about that). It seems it's easy to do. Not clear what data rates SL can sustain, though.