The Green Bay Packers assistant head coach/inside linebackers coach is Winston Moss, who's worked with Mike McCarthy in Green Bay since 2006. They were both on the coaching staff in New Orleans, so they certainly know each other well. And McCarthy would apparently like to keep him around as long as possible since he was the only defensive coach not fired after the 2008 season. He's also done a good job coaching the inside linebackers.
But not all assistant head coaches are really a true No. 2 man writes Dan Pompei at the National Football Post:
Not all of them are true assistant head coaches. Some are glorified position coaches who have the title only because their teams had to offer it to them as inducements. And the role of each one is a little different based on individual circumstances and relationships. But most of them have responsibilities beyond the responsibilities of most assistants.
One assistant head coach, who I believe only received the title as an inducement, is former Packers, and current Cowboys wide receivers coach Jimmy Robinson. He was hired by the Cowboys shortly after the Super Bowl, and McCarthy isn't known for letting his coaches interview with other teams. I expect the Cowboys had to add the assistant title to make it an opportunity McCarthy knew Robinson couldn't pass up.
I don't think it's just a title for Moss, who's interviewed for head coaching vacancies with the Raiders and Rams. I could see him standing up to McCarthy, if necessary, and coaching beyond the inside linebackers on the practice field.