I spend most of my time over at "F150online" and the rage over there lately is for guys to trade in their trucks for a new Ford Focus because of the poor gas mileage.
Don't expect great mileage out of your truck -- it just doesn't happen. I seriously question someone getting 17-18 MPG with any consistency on one of the new bodystyle trucks with the 5.4 engine. They are just such heavy vehicles that it's tough to do unless you've got a feather-touch on the gas pedal.
I've never even checked the gas mileage on my F-150 since I wouldn't want to let knowing how bad it really is keep me from enjoying the sound of my MagnaFlow cat-back system as I pull away from a stoplight! Of course, I have the luxury of only using my F-150 as a weekend cruiser, so gas mileage really isn't all that important to me.
Anyways, welcome to the F-150 family -- they're really nice trucks as long as you're not counting on them to be daily drivers that deliver great fuel economy. Get over to "F150online" and sign up -- lots of great folks with a wealth of knowledge about the vehicles!