As of S03EP01, does walker blood have an effect on open orafices/wounds?

I think that they may be modeling the initial fear and then subsequent relaxing of fears in the same way HIV/AIDS was viewed initially.

At first it was believed that being around people that were infected would cause you to be infected as well (drinking after someone, etc), but this was eventually dispelled.

From the Dr. at the CDC, Rick learns that they have all been infected and carry a dormant form of the zombie disease, but being bitten, scratched or dying accelerates the progression.

In the case of Herschel and his leg, we see that if you are able to remove the infection quick enough, one can survive a bite.

This may have been different in season 1, where you see several examples of people committing suicide or dying without turning. In Season 3 and going forward, I think its understood that you don't turn till you are attacked and/or die.