Do you think leadership is more important or less important in today's flatter, team-based organizations?

The answer to this question is going to change depending on who answers it.  The support is the most critical part of the answer, since either answer can be potentially correct.  
Personally, I think having a good leader is just as important as it ever was.  Whether a company is structured around a "team" based concept or not, that team needs to be led.  That's why sports teams still have coaches and aren't run by the players deciding who is going to play when and where.  The coach has the big vision in place, and a good coach knows how to get the most out of each player for the benefit of the entire team.  Businesses and companies are the same way.  
Take Valve for example.  By all appearances, the company is as team structured as a company gets.  Employee desks all have wheels on them, so that employees can move from coworker space to coworker space in order to teamwork on whatever project that they feel like working on.  If somebody has a good idea, and other people want to help with the project, then they are free to do that.  The company doesn't track vacation days or sick leave either.  Each employee is expected to operate for the benefit of the company as a whole, but the company does have a leader.  Gabe Newell is the CEO and founder of the company, and he is considered one of the strongest leaders in the entire video game industry.  He is absolutely monitoring the progress of his company, and he steers it toward success as necessary.  His vision and leadership created his highly successful company, and his company falls apart if the other leaders in it aren't hiring the kind of employees that can operate within Valve's unique business model.  
Yes, absolutely, a leader is still crucial in today's team-based organizations.