It mirrors Rally's Flow-Pull-Innovate agile adoption model, but there was a catalyst that distilled what surely would have taken months of meetings and decision making down into just several hours.
If you have some teams with some success under their belts, consider asking to schedule a meeting with management (as high up as you can) for a meeting where you will share the agile success metrics (of your team and others in the industry) plus a short overview of agile (what problems it solves, how it works, and culture changes/failure modes). Make sure to have the ScrumMasters and Product Owners on those one or two successful teams present to answer real life questions of what went well, not so well, and lessons learned along the way.
In my case, our meeting ended with all of management: convinced agile works, understanding the road is long and hard, deciding on next logical steps (which projects, what training, etc.).
But lead with the success you and others have had. Executives need to know why they should care and what's in it for them, then tell them more.