Currently creating it in PhotoImpact from Ulead. The art comes from various sources such as the fill art from Symbol Set 2 (CC3) and the buildings' roof textures from a texture collection I've had for a while as well as some of my own work using Photoshop or Genetica Pro.

And... you're right, it's going to be a mammoth job

I've actually made quite a collection of custom textures using Genetica that I keep meaning to assemble and put up for download here for anyone that wants to use them. I'll get around to that sometime soon, hopefully.