Lego CAD software Answered
I just thought that the Lego-ers (?) would like to know about Ldraw, open source CAD software specifically for designing Lego models.
Have a look at this BBC News story to see a chap who built a model of St Pancras' railway station to a scale suitable for minifigs.
Of course, you may not wish to go "open" with your software (I doubt that Ldraw has been as thoroughly debugged as Firefox, for instance), so you may with to use the official Lego CAD software.