Dropdown field 'default' I have a field of the dropdown fieldtype, let's call it myDropdown. 

myDropdown has a list of options, one of which has the "default" checkbox checked.

If I use a front-end form to submit a new entry, but the form does not contain the myDropdown field, will Craft insert the value designated as the the default into that field, or will it just leave the field empty?
Yes, if you don't include the dropdown field in your entry form, Craft will store the default value for that field, if one has been set. If no default value has been set, then the value will be null.