From the "Make Me Pulse" blog (of Nicolas Rajabaly & Antoine Ughetto) there's a quick example of how to use serialized values with SimpleXML:

Serialize is useful for storing or passing PHP values around without losing type and structure. But if you want to serialize a SimpleXml object, you will have some problem on unserialize with the error. [...] Replacing SimpleXMLObject with stdClass is a good idea but in this solution we loose all of attributes, and how can we make simplexml->xpath after?

The solution? Serializing the XML content and then outputting it from the SimpleXML object as an XML string (to be stored). This process is reversed when the data is needed back out.