Åkestam.Holst [1] in Sweden turned a building in Stockholm into a giant speaker to advertise their Hi-Fi client "Pause", which just happens to be located in that same building. Clever.

Tagline: Extraordinary powerful store
[via Researcher [2]]
Links:
[1] http://fabriken.akestamholst.se/
[2] http://researcher.se/archives/2005/12/uppkopplat/
[3] http://technorati.com/tag/Ambient & Guerrilla