There is no evidence that a dog infected with Lyme can transmit the disease to a human. But a dog that has a lyme infection is a concern because animals can carry ticks and "drop" them in areas where people can "pick" them up.
Lyme disease can be transmitted to a human by a bite from an infected tick.
Lyme disease can be transmitted to an unborn child by a mother infected with the bacteria.
Lyme disease can live in stored blood (blood banks).
Because Lyme Disease is a relatively "newly" identified infectious disease, I think it's safe to say we don't know alot about it and nothing should be taken for granted. Although the Center for Disease Control says "there's no evidence" to support the transmission of a lyme disease by such and such, that's not to say it can't happen...they just don't have evidence that is has!