As Apple states in its iPhone Safety Guide:
Driving and Riding Safely
Use of iPhone alone or with headphones (even if used only in one ear) while driving
a vehicle or riding a bicycle is not recommended and is illegal in some areas.
Certainly it would seem prudent for Apple not to even suggest that the new headsets be intended for use in the iPhone. In particular, the in-ear construction totally blocks external sounds, unlike the current ear phones. The liabilities could be onerous.
Obviously, one could claim that they would never drive with using the headphones in both ears. However, just walking out of your home with them on, you put yourself in jeopardy if you are deemed liable for causing an accident while wearing headsets.
Sounds scary doesn't it. However, even the "USA Track & Field", the national governing body for running, has banned the use of headphones and portable audio players like iPods at its official races, "basically (because of) an insurance issue.