There's no explicit restriction on using inline scripts with XHTML, but any occurance of "<" or ">" in the script code will cause a problem.
To get around that you can use a CDATA section:
Code:
<script type="text/javascript">// <![CDATA[
document.write('<a style="color:white" href="'+'mailto:'+'cg9com'+'@'+'juno.com'+'">'+'cg9com'+'@'+'juno.com'+'</a>');
// ]]></script>
Text within a CDATA section will be ignored by the XML parser.
Also, if you use document.write() to generate HTML dynamically, you need to be sure that HTML is valid.