Capture and cancel textarea keydown event in NS4 and up?

I'm trying to restrict the length of textarea input fields in JavaScript. No problem in IE but Netscape is giving me grief - seems to be ignoring the event entirely. I need to cancel the event before it occurs so new text is not added to the textarea. I'd also like to ignore delete key, backspaces and arrow keys (i.e. only captureEvents that actually lengthen the textarea size). This also has to work with the Japanese character set.

so far...

<SCRIPT>
document.campaign_data_form.campaign_desc.captureEvents(Event.KEYDOWN);
document.campaign_data_form.campaign_desc.onkeydown = checkLength(document.campaign_data_form.campaign_desc,'Campaign Description',400);
</SCRIPT>

and...

function checkLength(field,desc,length) {
//used with keydown to limit the length of an input field
     //ignore delete, backspace and arrow keys
     if (!(self.event.keyCode == 8 ||
            self.event.keyCode == 46 ||              
            (self.event.keyCode >= 37 && self.event.keyCode <= 40)              
               )) {
          if (field.value.length >= length) {
               self.event.returnValue = false
               alert (desc + ' cannot be more than ' + length + ' characters ' + self.event.keyCode + ' ' + field.value.length);
               if (field.value.length > length) {
                    field.value = field.value.substr(0,length);
               }
               return false;
          } else {
               return true;
          }
     } else {
          return true;
     }
}

As usual, please post answers as comments so I can choose which answer fits best.