My builder wants to trim my new house with MDF instead of solid timber. It this a good idea?

MDF (medium density fibreboard) is basically pulped wood that has been glued back toghether and pressed into shape.  Termites don't really like it because of the high glue content.  It is a counter intuitive thing, but you are actually better off using the trim that termites really like to eat.  That way when they do attack, you have a good chance of finding the damage quickly (like with the vacuum cleaner leaving a dent) before too much bad stuff can happen where you can't detect it. 

You want all the timber that's easy to see or bump to be just about as susceptible to termites as is possible.  All the stuff that's deep in the walls can be resistant, but not the other way around.  Termites often eat MDF only a tiny bit while completely wrecking the normal timber behind it and it just makes their activity much harder to find.  MDF looks smooth and paints well, but a clear coat over timber is much prettier and it doesn't add a lot to the cost, well not compared to its early-warning value.  Just think of it as another part of your termite management defences.