Great question as to where to draw the line. Maybe "don't endanger the health of others" is a useful rule of thumb. I don't mind/care if people smoke in their own homes, but don't want them smoking near my children in a park.

To show another extreme, the recent California bill making it not a felony to give HIV to others, or to donate blood if you're HIV-positive is where it seems like the government is being too hands-off.