To get xForms to work with IE8 compatible mode u need to change a function in xformedit.js.
U can find the file at /edit/ui/javascript/. Look then for this function:

function formPopulateForSubmit( oForm )
{
   fieldPropertiesHideAll();
   formContent.value = xFormControl.innerHTML;
   return true;
}

Replace it with this function instead:

function formPopulateForSubmit( oForm )
{
    fieldPropertiesHideAll();
    
    var cnt = xFormControl.innerHTML;
    var regExpInput = new RegExp(']+>');
    var inputTemp = regExpInput.exec(cnt);
    
    while (inputTemp != null) {
        var regExpType = new RegExp('type=text|type=radio|type=checkbox|type=submit');
        var typeTemp = regExpType.exec(inputTemp[0]);
        var type = (typeTemp != null && typeTemp.length > 0) ? typeTemp[0] : '';
        var str = inputTemp[0].replace(type, '');
      
        if (type == 'type=text') {
            type = '';
            var regExpGuid = new RegExp(' {[a-zA-Z0-9]{8}-[a-zA-Z0-9]{4}-[a-zA-Z0-9]{4}-[a-zA-Z0-9]{4}-[a-zA-Z0-9]{12}}');
            var guidTemp = regExpGuid.exec(str);
            if (guidTemp != null && guidTemp.length > 0)
                str = str.replace(guidTemp[0], '');
        } 
        else if (typeTemp == 'type=submit') {
            var regExpOnCLick = new RegExp('onclick=[^\f\n\r\t\v]+');
            var onCLickTemp = regExpOnCLick.exec(str);
            
            if (onCLickTemp != null && onCLickTemp.length > 0) {
                str = str.replace(onCLickTemp[0], '');
                type = onCLickTemp[0] + type;
            }
        }      
        
        str = str.replace('<INPUT', '<$MGL$ ' + type);
        cnt = cnt.replace(inputTemp[0], str);
        var inputTemp = regExpInput.exec(cnt);
    }
    
    while (cnt.indexOf('<$MGL$') != -1){
        cnt = cnt.replace('<$MGL$', '<INPUT');
    }
    
    formContent.value = cnt;
    return true;
}

Don't forget to empty IE8 cache and temporarily Internet files.