Metal can be an important component in timber frame construction especially when tension loading, connection to foundations, etc. come into play.
I think on the scale of a small house / barn the use of steel can be minimized to almost nill based on the design (nails, some bolts to the FND, etc) - and Tim's point about embodied energy and technology is valid.
I really like where that discussion could go - we could have its own thread for it (we have, sort of) - and something I've oft debated with 'traditional' woodworkers (ones who can sometimes border on luddites (in a good way) - is the question of is the Japanese plane a 'truer' simple man's tool over the western style plane?
In the Japanese model - one need only contract for the blade, from a local smith - the balance of the plane can be made by the carpenter - and adjustment and even rebuilding can happen by the hands of the woman or man who uses the tool. (this assumes one has all the prerequisite chisels and even other planes...)
In the western style (were talking iconic metal hand plane with wooden handle here) - one needs to invest in casting equipment, molds, metal finishing equipment, hardware, dies, taps, etc. etc. And should the plane be damaged or need adjustment - common tools used to work wood can not help you... the embodied technology and energy and infrastructure needed to support and create the tool is far greater...
Which leads one down the path of discussing embodied energy, technology, etc. from the ax to the pit saw to the water mill to the electric table saw to the chain mortiser to the Hundegger to the laser cutting timber assembly machine on the international space station to the particle collider.
I like the image of a carpenter walking out the back door and plucking a steel section from the misty and pastoral landscape, dragging it into the shop with oxen, then using a modified hand powered boring machine to mill, drill, and shape the steel for connecting timber together.
That all said, steel plate joinery or hidden steel joinery is IME more expensive than traditional wood to wood joinery when the loading and use of the structure do not require the properties steel brings to the equation.
But, when you are a hammer, everything starts looking like a nail... and I'm sure a dedicated truss shop with plate cutting / drilling capability could prove me wrong.
