After playing around for a while I discovered that it is some sort of bug. The following code does work in 1.5 on Linux.
<html>\n<head><title>Test Radio</title></head>\n<body>\n<form id='MyForm'>\n <input name='MyRadio' id='MyRadio' type='radio' value='Y' checked />Yes <br />\n <input name='MyRadio' id='MyRadio' type='radio' value='N' />No <br />\n</form>\n\n<script language='JavaScript' type='text/javascript'>\n if (document.getElementById("MyForm").MyRadio[0].checked) {\n alert("hello");\n }\n</script>\n</body>\n</html>
The only difference being I put the type after the name and ID.
Seems to be this one [link|http://bugzilla.mozilla.org/show_bug.cgi?id=183940|Bugzilla].
Jay