identity constraints and xsi:nil

If an element selected by the field  of an identity constraint has 
xsi:nil='true', is the value treated as missing? For example, is the 
following instance valid, given the schema.

I think this should be valid since the xsi:nil attribute on the uid 
elements would be equivalent to the elements missing for the purposes of 
identity constraints. Either way 3.11.4 of Schema part 1 could use some 
clarification around xsi:nil.

Tim



schema
------
<?xml version="1.0"?>
<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">
         <xsd:element name="root">
                 <xsd:complexType>
                         <xsd:sequence>
                                 <xsd:element ref="uid" 
maxOccurs="unbounded"/>
                         </xsd:sequence>
                 </xsd:complexType>
                 <xsd:unique id="foo123" name="uuid">
                         <xsd:selector xpath=".//uid"/>
                         <xsd:field xpath="."/>
                 </xsd:unique>
         </xsd:element>
         <xsd:element name="uid" nillable="true" type="xsd:anySimpleType"/>
</xsd:schema>

instance
--------
<?xml version="1.0"?>
<root xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
xmlns:xsd="http://www.w3.org/2001/XMLSchema" 
xsi:noNamespaceSchemaLocation="idF018.xsd">
         <uid xsi:nil="true" xsi:type="xsd:string"/>
         <uid xsi:nil="true"/>
</root>