I've seen it done in some PC mods, There was one in Popular Science a couple of months ago, but I think its also on mini-itx.com.

As for the router vs hub, go with the router, since they usually double as switches, which helps especially if your network is going to grow (or if its going to manage a lot of bandwidth).Size isn't going to differ too much

Depending on how long you plan on keeping the computer running you could just add a relay to the Main PSU to control power going to the router/hub's power supply. It's a common idea in the Power CC ring.