I'm an EX minus 3 with no direct reports and no budget. I'm not a manager. Does that mean I'm not a leader?
I'm a software developer so I come at this question with an information architecture approach. I think there are (at least) three key dimensions or facets of leadership in play in our workplaces.
The first is people management - what I'll call the "leadership relationship" is supervisor/subordinate. Supervising is purely an HR function - performance management, learning plans, managing leave, travel, training, etc.
The second is action management. Whether tasks are ongoing or part of one-off projects, there's usually a team involved, so my leadership relationship for this one is team lead/team member. It's the team lead's responsibility to ensure the successful delivery of the project/service/whatever whether the members report to him or not.
The third is competency building. The leadership relationship here is mentor/protegé. The protegé seeks out guidance from the mentor as needed. The mentor coaches the protegé through "stretch" tasks and spreads knowledge via conversation, e-mail, lunch-and-learns, etc. In addition to technical or operational competencies, this can apply to values and ethics, strategic thinking, engagement, and management excellence. The beauty of living in the future, as Wil Wheaton says, is that social networking tools make it easier than ever for the mentor/protegé relationship to cross the boundaries of work units, departments and agencies, and even the government itself.
I think that the larger the organization, the more overlap there is among these facets in any given working relationship between two people. I work in a relatively small IM/IT shop - we don't have the luxury of a lot of specialists. In many projects, the team lead needs to wrangle multidisciplinary resources that don't report to her, and it requires a culture where supervisors don't hoard their subordinates for their own projects.
Maybe the most novel point in all of this for me: it is possible to have team leaders who don't supervise or mentor, supervisors who don't mentor or lead teams, and mentors who don't supervise or lead teams. I don't think it's ideal from an operational standpoint, but we don't work in an ideal world. Supervisors who don't lead teams is the tricky one, as it requires phenomenal communication with team leaders to manage performance effectively.
In terms of building leadership capacity in the context of public service renewal, I think it's incredibly important not to mistake managing for leading. It's easy to let the (very real) need for good people management to blind us to the importance of the other key leadership competencies.
This isn't a fully-formed theory of leadership by any means, but it's something I've thought about off and on over the last several months as my unit undergoes a re-organization. I'm putting it out there, as so often happens, because of a short conversation on Twitter, and I'm hoping it will prompt a conversation in its own right.