At most of my Scrum training classes, people ask about how to scale agile at their company. When coaching at a company, what we are doing is typically the #1 need for scaling - getting better.
As Woody Zuill said so well at the Agile Open SoCal conference last month, "People ask how to do it right in the large, when they're not even doing it right in the small."
Yes, there are methods, tools and techniques for doing agile with 10+ teams, or multi-team projects, or distributed team members. But those practices won't solve the most common problem - lack of organizational alignment, buy-in and support (and from top to bottom). Most of the issues I find are with mid-management handling the change that come with introducing agile. But this is the same challenge that happens with introducing any change. The book Leading Change reinforces this point with 10 years of stories around introducing change at difference organizations.
My experience has been that you'll be much better off getting one or two teams excellent. And by that, I mean success that everyone can see, and that is obvious. Nothing wins arguements and gets support like success, and this helps mid-management's willingness to go out on a limb with this new thing that will surely: raise more questions, change they way they do their job, move them out of their comfort zone, upset some of their reports, make mistakes. Help make that gamble for them as easy as possible.
And keep in mind that we're in the Late Majority with agile adoptions. These are companies, customers, and managers who generally do not like change, resist change, and have been at the same company (and maybe job) doing things mainly the same way for 10, 20 or 30 years.