Ahh well, I'll add to it, since I wasn't on here when it started. I have a PhD in mining rock mechanics.
My business card takes the following form.
First Name Last Name
BE (Hons) Mining, PhD, Professional Affiliations
I don't use the Dr in front of my name on the card.
I'll only use it if I'm filling out forms - I don't introduce myself as Dr so and so in a social environment. Too wanky and invariably leads to the question about what field of medicine I practice in.
Although, in a professional environment its quite formal.