ZF-8911: wddx_serialize and utf-8 data will not work correctly
The wddx_serialize function always encodes to utf-8...even when the data is already utf-8. Therefore you must decode to iso-8859-1 before wddx_serialize, or else your data gets "double" utf-8 encoded. This means wddx_serialize can't really serialize utf-8 encoded strings. If this is a PHP bug I don't know...but ZF should be aware of this and maybe implement this workaround.
Try wddx_serialize with a string in utf-8 encoding containing some special characters that are not in the standard english alphabet. The resulting wddx xml document will not be correct.