well there's another way of doing it. It's without the <form:radiobutton> tag, by using <spring:bind> tag
Code:
<spring:bind path="known">
<input name="${status.expression}" type="radio" value="foo" <c:if test="${status.value == 'foo'}"> selected</c:if> >
</spring:bind>
since you want to check for some other conditions you can try something like:
Code:
<spring:bind path="known">
<c:if test="${status.value=='foo' && myBoolean }">
<c:set var="selected" value="true"/>
</c:if>
<input name="${status.expression}" type="radio" value="foo" <c:if test="${selected}"> checked</c:if> >
</spring:bind>
hope this might help you