According to Musk, Tesla's Jerome Guillen is already driving and testing the Tesla semi.

Apparently Guillen leads the team that's working on the Tesla semi.

Master Plan Part Deux puts the semi unveiling to be sometime next year. No word on when or if it'll ever go on sale for public consumption. From the plan:

"In addition to consumer vehicles, there are two other types of electric vehicle needed: heavy-duty trucks and high passenger-density urban transport. Both are in the early stages of development at Tesla and should be ready for unveiling next year. We believe the Tesla Semi will deliver a substantial reduction in the cost of cargo transport, while increasing safety and making it really fun to operate."