насколько я знаю по стандарту пишется :
<?xml version="1.0"?> а не <?xml version="1.0">
пример -
a = "<?xml version='1.0'?><doc xml:base='http://example.org/today/' xmlns:xlink='http://www.w3.org/1999/xlink'><head><title>Virtual Library</title></head><body><paragraph>See <link xlink:type='simple' xlink:href='new.xml'>what's new</link>!</paragraph><paragraph>Check out the hot picks of the day!</paragraph><olist xml:base='/hotpicks/'><item><link xlink:type='simple' xlink:href='pick1.xml'>Hot Pick #1</link></item><item><link xlink:type='simple' xlink:href='pick2.xml'>Hot Pick #2</link></item><item><link xlink:type='simple' xlink:href='pick3.xml'>Hot Pick #3</link></item></olist></body></doc>"
newXml = new XML(a)
trace(newXml.firstChild)