Спасибо, madgnome! Я думал, что поделюсь рабочим кодом.
<Target Name="UpdateWebConfigVersion">
<GetAssemblyIdentity AssemblyFiles="lib\foo.dll">
<Output TaskParameter="Assemblies" ItemName="fooAssemblyInfo"/>
</GetAssemblyIdentity>
<XmlUpdate XmlFileName="src\Web\ServiceModel.Extensions.config"
XPath="//extensions/behaviorExtensions/add[@name='silverlightFaults']/@type"
Value="foo.ServiceModel.Extensions.Behaviors.SilverlightFaultBehavior, foo, Version=%(fooAssemblyInfo.Version), Culture=neutral, PublicKeyToken=XXXXXXXX"/>
</Target>