nillable values for attributes

Is there any discussion for allowing attribute values to have a
"nillable" value.  I know that as XML Schemas currently stand, only
elements may be nillable.  Plus, is there a work around for this?  Say I
wanted an attribute with a Boolean datatype, but it could also be
nillable. 

This is what I would like:
<xs:attribute name="test" type="xs:Boolean" nillable="true"/>

So an instance document's attribute value could contain any of the
following and be ture:

<someElement test=""/>
<someElement test="true"/>
<someElement test="false"/>


Thanks,

Sean



Sean Gerety
Extreme Logic, Atlanta
sgerety@extremelogic.com