jQuery: Checking for disabled attribute and adding/removing it?

I select all input elements of a form like this:

var fields = $( form + " :input" ).not( "button" );

How do I check if any of these inputs has the disabled attribute set and remove it when present?

Also, I need to add it after is has been removed (and serialize the fields in between), is there an elegant way for this? Something like toggle but for attributes?