Brad Humphreys at the IJSF blog has written extensively on the reluctance of the NHL to allow the Phoenix Coyotes to move to Southern Ontario. See here, here and here, though there are obviously many more as the last one listed is Part X. The resistance to the move is motivated by protecting the territorial rights of the Toronto Maple Leafs and Buffalo Sabres.
However, I believe there is a pretty strong secondary motive to the reluctance to a move: the relatively new arena that the Coyotes play in Glendale. Glendale built an arena for the Coyotes in 2003. The arena is only for hockey, as the Phoenix Suns play in US Airways arena (formerly American West Arena; that name change cost me a point in a recent trivia contest). While some teams certainly care about some future team invading their territorial rights, they are all interested in eventually having an arena built for the team by the local municipalities. I do not think Nashville has to worry about another hockey team moving to their city, but they will probably want the city to build them a new arena at some point.
If the Coyotes move out of Arizona, the next time a team threatens to move unless they get a new arena, opponents will have a pretty convincing retort:
Team: "Build us a new arena or we will move to Albuquerque."
Opponents of new arena: "Well, if we build the arena, we will spend $300 million, then the team will play there for five years and then move to Canada. Look at what happened to Glendale."
One way to test this theory is to see if the league would restrict the team from moving to a location without a problem with territorial rights such as Houston, Kansas City or Winnipeg. Of course none of those markets would be nearly as valuable to an owner as Southern Ontario.