One of my favourite examples of an approach to scaling agile structures is detailed in this paper by Henrik Kniberg and Anders Ivarsson talking about Spotify's model of Tribes, Squads, Chapters and Guilds (in fact the guys at Undercurrent, whom I talk to below, link to it from their site). Spotify has scaled more rapidly than most businesses but this structure is specifically designed to maintain an admirable level of agility across 30 teams and 3 cities.
The basic unit of development at Spotify is the 'Squad', a small, nimble, multi-discplinary, automous self-organising team that is designed to feel like a mini-startup. Squads are co-located and focus on specific areas of the product or service, incorporating all the tools and skills they need to take an idea from design to test to release and production. Much like Amazon's Two-Pizza teams, Squads are clearly focused on a specific task and KPI.