Mike,
After a couple of minutes and the engine starts to build some heat, does the problem start to clear up? I had a similar problem with my 03 Ex. On the initial start-up of the day, it had no power, to the point I would not go much above idle throught most of my subdivision because it had no power. Once there was a bit of heat in the heads it would clear up. This is a common problem with the 6.0L that Ford has called "stiction" (Stick-shun). The 6.0L has a HEUI injection system (Hydraulicaly operated, Electrically actuated, Unit Injector) it uses high pressure engine oil to create the pressure to inject the fuel in the cylinder. On an IDI, there is an injection pump that pressurizes the fuel, which in turn causes the injector to "pop" and atomize the fuel into the cylender. In the HEUI system the lift pump moves fuel into the injector at a "low" (~70psi) pressure. There is an HPOP in the valley in the top of the engine that pressurizes the engine oil and pushes it into the "top" of the injector. (~500psi to start and upto ~4000psi under heavy load). This engine oil pushes on a piston inside the injector, which pushes on an intensifier that increases the pressure 3:1. When the pressure is high enought the injector then "pops" and atomizes the fuel into the cyl.
What happens as the injectors age, the engine oil in them, when it is cold, is thicker and the injector has more diffuculty in using this oil to pressurize the fuel. So you have a ver poor running truck for the first few minutes in the morning, or after a long time of sitting. Ford released a PCM/FICM update that uses "inductive heating" to help to warm the oil in the injectors before startup. How this works is, there is a small electrical solenoid in the injector that controls the oil flow in the injector. Upon startup, while the WTS light is illuminated, the FICM cycles the solenoid which causes a small amount of heat to be built inside the oil area of the injector. This help to eliminate the "stiction" issue and the cold/no power issue. If you have this update, when you turn the key to the run position waiting for the WTS light to go out, you can hear a clacking sound as all of the solenoids are being cycled. This will be for just a few seconds. One trick I found to help until I was able to get it in for service, was to plug the block heater in no matter what the temp was. It would start and run fine if I did this, since the oil was already somewhat warm. This flash would have to come from a Ford Dealer, and my local dealer said they charge about $100 to so an update when it is out of warrenty. Luckily my still had a little warrenty left when I had the problem..
As for the crappy mileage. Have you changed the Fuel Filters lately? A 6.0 is very picky about maintenance. I change my oil a 5k and fuel filters at 10k with NO EXCEPTIONS!!!!! Also, use ONLY Motorcraft oil and fuel filter. Some of the aftermarket filters have caused problems in these engines. It is worth a couple of extra bucks on a filter to help protect your $10k engine.
Also, if you could fill in your Signature with your truck's info so we know what you have, it is easier to answer your questions that way. Click on the UserCP link in the blue bar at the top of the page.
I hope this helps, and if this does not sound like the problem, let us know and we can go from there.