well...
i would look at the clutch switch, as stated before.
check the wires to your starter, check grounds.

you can rule out the ignition switch for now since it works in other positions.

you are sure the battery has a really good charge, right?
also your battery connections are nice and clean right?
did you check the crank sensor yet?

NY State Trooper: So what makes your car so special to have SVT all over it? Me: Er...It was made by Fords SPECIAL Vehicle Team?