Addendum to my nominal types: For the 'fake' nominal types (you can't put a symbol on a number!) I like to make those fields optional, which makes the type less of a lie. "If this number did have a [unit symbol] property, the value would be 'USD'". That will still prevent accidental USD <-> CAD conversion, but allows converting to/from regular numbers without explicit casting.

A pile of thoughts on the subject from back when I first came up with it: http://www.nuke24.net/plog/32.html