With jQuery, how do I capitalize the first letter of a text field while the user is still editing that field?

Just use CSS.

.myclass 
{
    text-transform:capitalize;
}

This will simply transform you first letter of text:

yourtext.substr(0,1).toUpperCase()+yourtext.substr(1);

I answered this somewhere else . However, here are two function you might want to call on keyup event.

To capitalize first word

  function ucfirst(str,force){
          str=force ? str.toLowerCase() : str;
          return str.replace(/(\b)([a-zA-Z])/,
                   function(firstLetter){
                      return   firstLetter.toUpperCase();
                   });
     }

And to capitalize all words

function ucwords(str,force){
  str=force ? str.toLowerCase() : str;  
  return str.replace(/(\b)([a-zA-Z])/g,
           function(firstLetter){
              return   firstLetter.toUpperCase();
           });
}

As @Darrell Suggested

$('input[type="text"]').keyup(function(evt){

      // force: true to lower case all letter except first
      var cp_value= ucfirst($(this).val(),true) ;

      // to capitalize all words  
      //var cp_value= ucwords($(this).val(),true) ;


      $(this).val(cp_value );

   });

Hope this is helpful

Cheers :)