The maxlength attribute on textarea is supported in Internet Explorer starting with version 10, Chrome, Firefox, and (current) Opera.
Note: when pasting content longer than the max-length, the content will be silently truncated. This behavior could be less desirable than the current one where the user simply receives a validation error. Maybe the HTML5 validation features can help in this regard.
Consider adding an option to disable the client-side MaxLength check on paste and text-input while still keeping the validation working.