Ideally you want to have some kind of trunking with a removable cover as no doubt over time you'll want to upgrade or add new connections. Does your current plasma have a HDMI connection? If/when you go HD, you'll need one.
Neat solution would be to chase this into the chimney breast, with the removable cover to the side of the breast (I'm assuming there is one?). With a large enough channel, you could re-plaster to the front of the chimney breast over the fire, and just thread the leads through from the side to an exit behind the screen. Big enough job though with complications of weakening the structure.
On another note, do you use the fire much? The heat isn't going to do a plasma any favours...
Leo