Serialization of Java 5 Enum to/from xml attribute doesn't work:
xstream.useAttributeFor(MyEnum.class)

unless SingleValueConverter is registered:
xstream.registerConverter(EnumSingleValueConverter.create(MyEnum.class));

below (and in attachment) is the converter code:

public class EnumSingleValueConverter<T extends Enum<T>>
extends EnumConverter implements SingleValueConverter {

    private Class<T> enumType;

    public static <V extends Enum<V>> SingleValueConverter create(Class<V> enumClass) {
        return new EnumSingleValueConverter<V>(enumClass);
    }
    private EnumSingleValueConverter(Class<T> newEnumType) {
        this.enumType = newEnumType;
    }
    public boolean canConvert(Class type) {
        return type == enumType;
    }
    public Object fromString(String str) {
        return Enum.valueOf(enumType, str);
    }
    public String toString(Object obj) {
        return obj.toString();
    }
}

it should be possible to add support for that in XStream facade so user wouldn't have to register anything.