With or without actual rotting, I imagine that whatever has infected the person did get into their saliva before they dried up, and as a result has been left in the zed's dry mouth. Bacteria, virus, whatever is probably still clinging to the teeth and has no problem surviving once it hits your bloodstream.
I would have to say body fluid contact, the mouth of said zombie would more then likely be covered in open sores and coagulated blood. The mouth is also one of our only appendages that is capable of brakeing the skin, introduceing contact. I would think more along the lines of 28 days later for infectious contact.
i do believe everyone is on the right track mouths carrie lots of virual infections, a zombies mouths dries out, but it bites some one then they have blood in there mouth which is moister allowing the virus to multiply and the process it repeated over and over. Also viruses can be dormant for days months years, if a scientist finds a site with bodies not matter how old they have to wear masks and gloves not just to protect the dig site, but also to protect from any old viruses that at there laying dorment. Think about it, if you get the flu virus in the morning, by early afternoon your feeling it.