I have created a custom object with lookups to various standard SF objects (Account, Contact, Opportunity, Lead and Case).  My Visualforce page contains a very basic form with all available fields set up as <apex:inputField> elements and this works as expected - as Visualforce.  Now my intention for this page is to make it available via Sites.  However, when I expose this VF page on my site, the lookups fail (and give me the error in the subject line).  It appears that some common SF components are not made available for Sites usage.  Is this true?  I hope I am just missing something simple.

This is a very basic proof-of-concept implementation.  The Sites profile has read/write access to my custom object, read access to all standard objects listed above and no custom extensions have been implemented yet so no additional Apex classes are necessary for this page.

My code is below:

<apex:page standardController="Foo__c" showHeader="false">
<apex:messages />
<apex:form >
<apex:outputPanel layout="block">
<apex:outputLabel for="subject" value="Subject"/>
<apex:inputText id="subject" value="{!Foo__c.Subject__c}"/>
</apex:outputPanel>
<apex:outputPanel layout="block">
<apex:outputLabel for="body" value="Body"/>
<apex:inputField id="body" value="{!Foo__c.Body__c}"/>
</apex:outputPanel>
<apex:outputPanel layout="block">
<apex:outputLabel for="case" value="Case"/>
<apex:inputField id="case" value="{!Foo__c.Case__c}"/>
</apex:outputPanel>
<apex:outputPanel layout="block">
<apex:outputLabel for="account" value="Account"/>
<apex:inputField id="account" value="{!Foo__c.Account__c}"/>
</apex:outputPanel>
<apex:outputPanel layout="block">
<apex:outputLabel for="opportunity" value="Opportunity"/>
<apex:inputField id="opportunity" value="{!Foo__c.Opportunity__c}"/>
</apex:outputPanel>
<apex:outputPanel layout="block">
<apex:outputLabel for="contact" value="Contact"/>
<apex:inputField id="contact" value="{!Foo__c.Contact__c}"/>
</apex:outputPanel>
<apex:outputPanel layout="block">
<apex:outputLabel for="lead" value="Lead"/>
<apex:inputField id="lead" value="{!Foo__c.Lead__c}"/>
</apex:outputPanel>
<apex:commandButton id="save" action="{!save}" value="Save"/>
</apex:form>
</apex:page>

 Any ideas would be much appreciated!