newTextField = Ext.extend(Ext.form.TextField,  {
    unitText : '',
    onRender : function(ct, position) {
        newTextField.superclass.onRender.call(this, ct, position);
        if (this.unitText != '') {
            this.unitEl = ct.createChild({
                tag : 'div',
                html : this.unitText
            });
            this.unitEl.addClass('x-form-unit');
        }
    },
    setUnitText : function(v){
        this.unitEl.set({html:v});
    }
});

Ext.reg('myTextField', newTextField);