Generally they vaccinate against Hep B if the parents are from a place where it is prevalent (2 or more % of the population have it) though since vaccinating everyone could eradicate it, many places are recommending it for everyone. There is a list on the CDC website
here of the places where it is prevalent. It might be a good idea to vaccinate babies in areas where a significant proportion of the population has arrived in the last generation from the areas where the disease is prevalent.
Hep B is passed by blood and bodily fluids. It can be contracted through risky behaviours (needle sharing) or normal behaviours (from an infected mum at birth, from sharing the toothbrush of someone who you had PROTECTED sex with, from having unprotected sex with one's long-term partner who just happened to get it through unprotected sex in a previous monogamous relationship). Most people who work with drug users (my mum included) are offered the Hep B vaccine, and many medical professionals have had it.
Babies born to an infected mother should be vaccinated within 12 hours to protect them from contracting the virus. The vaccine has been shown to be effective for at least 23 years in studies. In the UK (where the government foots ALL the medical bills, for both vaccines and HepB treatments) all women are offered HepB testing during pregnancy and all babies born to HBV+ mums are vaccinated at birth.
HTH
Bx