This is the modern age, and preparations for 5 million people include a new wrinkle: mobile communications. Think of it, 5 million people texting their friends with photos of 5 million people. I've seen estimates of more than 1.4 billion mobile messages are expected to be delivered nationwide on Inauguration Day...
This doesn't count all the people trying to call each other to figure out where they are in the crush of humanity. Tons of emergency calls are a given for the statistical certainty of injury or illness.
Which brings us mobile, mobile communications. The main providers have set up an encampment on the east end of the mall with temporary cell towers:
I don't know how they tie back into the rest of the network, whether they tapped into a trunk or if they have a satellited dish hidden behind the fence, but the fact that such a resource can be deployed on a temporary basis is pretty damn cool.
Another note on Inaugural Infrastructure comes from the "Everybody poops" department. The Great Wall of Portapotties has been built from the Capitol for as far down the Mall as I could see. Photos don't do it justice, the sheer volume of plastic boxes is almost overwhelming. Sure, they're building huge scaffolding to hold up huge TVs, the news centers are being built, but the line up of crappers, that's how you know you live in a modern age.