Lard (or other fat) as a subsitute for butter in desserts for someone with a dairy allergy? My wife is currently unable to consume any dairy as our breast-fed baby has a milk protein allergy. When reading many dessert recipes, many of them call for butter. For example:

http://smittenkitchen.com/2008/04/shaker-lemon-pie/
or http://chefmichaelsmith.com/Recipe/cinnamon-rolls/


I know pie crusts for example can be made with either, but can they always be substituted?
When would I not want to use lard over butter?
What ratios should I use to convert the recipe?
Is there some other milk-protein free alternative that would work better?
Recipes for which you can substitute margarine for butter can have non-dairy margarines substituted instead - you have to check the ingredients list, most margarines contain milk solids. Here in Australia, my dairy free friends and I use Nuttelex, but that's not available in the USA.