Why Plant Churches in Global Cities?

In the following promotional video for Redeemer Church Planting Center Tim Keller and other church planters explain why planting churches in the world's big cities is so crucial.

RCPC: A Vision for Church Planting from Redeemer Video on Vimeo.

Keller's emphasis on cities has certainly caught my attention over the last few years. I'm still trying to figure out what to do about it on a personal level. ;-)