How to make US date format as the default when saving to a model in Rails 3

Question

The US date used to be accepted/parsed correctly, but not anymore in Rails 3. The %Y-%m-%d is accepted but not %m/%d/%Y.

g = Grant.new
g.budget_begin_date = '12/31/2010'
#g.budget_begin_date returns nil
g.budget_begin_date = '2010-12-31'
#g.budget_begin_date returns Fri, 31 Dec 2010 00:00:00 UTC +00:00