Did I hear DrupalXML library?
But seriously, simpleXML is great. I'm sure I'm not the only one that would like to start taking advantage of it. If we want to continue supporting php4(which I think is necessary) and we want simpleXML then maybe we should write our own library that degrades to something like the system linked by Karoly. That way we have a common interface and we can take advantage of the benefits of simpleXML. And if we take some care in designing the library it will make conversion to simpleXML at php4 EOL fairly easy.