A lot of things going on here; so, I'll leave the interpretation up to you. Roughly, I built the two cityscapes in Blender. In Photoshop, I added steam from the buildings' roof-top units and fog, along with some other hand work to fix things that were bugging me. Then applied a number of filters to degrade the scene to a point where I was happy with it.
I'm really liking the texture (you have to zoom in to see that very well) on the buildings. Reminds me of ink sketching and watercolor.
The final image is 24in x 38.66in @ 150ppi. You can see the zoomable version here:
Process in a little more detail:
I started by constructing the cityscape in blender, using a particle system. If you ever have a need, Andrew Price posted a brilliant video tutorial that makes it pretty easy:
Once I had a basic city, I duplicated it and turned it over. I'm sure this is an easy thing for many people out there, but it took me a while (and a bit of trial and error) to figure out how to turn a particle system upside down. I'm still not completely sure I understand what I did. Let's call it dumb luck.
Here is a piece of the render. Though you cannot really see it in this size, the render is very grainy. I didn't want to waste a bunch of time rendering with loads of samples to get a perfectly clean image. I planned to dirty it up anyway.
After a *LOT* of trial and error (and a lot of useless google searches), I figured out how to make Blender give me a depth map. I wanted to add a bit of atmospheric effects, and I knew it would be easier if I had a depth map to work from. There is probably a way to do the kind of atmosphere I wanted with Blender, but since I started using the Cycles engine, I have had to relearn a number of things. Atmosphere is one that I have not yet figured out.
For anyone who is interested and has a similar need of a generated depth map, here is my composite node configuration. If anyone has a more efficient or elegant solution - PLEASE let me know. I apologize for the weird image here - I used my iphone in panorama mode to grab this so I could remember how to do it again.
Basically, I took the Render Layers Z Depth to a Tonemap node, then sent that to the Composite node. That allowed me to save the depth map out to an image. Other solutions I found involved convoluted processes, including endering in code in the text pane. It all seemed too much, since I could  see the depth map easily by clicking a few buttons. I figured there had to be a way to use the composite nodes for a better and easier solution. So, render your scene, save your image, then set up this composite node configuration, and you can save the depth map out for the scene you just rendered. The Tonemap node is set to Simple. I think I left the other settings at their default - just in case, they are: Key 0.180, Offset 1.000, Gamma 1.000.
Off in Photoshop, I started by adding a levels filter to make the buildings pop a bit. I also added a star field in the background - which turned out to be a waste, as my later edits ended up making the starry areas unrecognizable.
I decided I wanted a deeper cityscape. Rather than go back into Blender and rerender the whole thing, I just duplicated what I had and added it in behind the existing buildings.
Another thing I have yet to figure out, when using the cycles engine - smoke and steam. So, I added that in Ps by hand. 
Using the depth map from Blender as a layer mask, I added in some atmosphere (rendered clouds and stretched them out, then set to screen) to increase the illusion of distance.
Then I started degrading the scene with different filters.
And here is the finished image
To zoom in to it, visit:
There is also a link to purchase this as a poster on that page (dare I hope anyone likes it that much??).