MsBuild target to rename Windows Phone XAP packages after building to include build configuration & assembly version number. Note that as far as I know, it's not possible to delete the original XAP file as it is required for emulator deployment & debugging.

MsBuildRenameXap.xml

<Target Name="AfterBuild">
    <GetAssemblyIdentity AssemblyFiles="$(OutputPath)\$(AssemblyName).dll">
      <Output TaskParameter="Assemblies" ItemName="MyAssemblyIdentities" />
    </GetAssemblyIdentity>
    <PropertyGroup>
      <XapFilenameWithoutExt>$(XapFilename.Replace('.xap',''))</XapFilenameWithoutExt>
      <OutputXapFilename>$(XapFilenameWithoutExt)_$(Configuration)_%(MyAssemblyIdentities.Version).xap</OutputXapFilename>
    </PropertyGroup>
    <Message Text="Removing all files matching with '$(XapFilenameWithoutExt)_*.xap'" />
    <ItemGroup>
      <OldXapFiles Include="$(OutputPath)\$(XapFilenameWithoutExt)_*.xap" />
    </ItemGroup>
    <Delete Files="@(OldXapFiles)" />
    <Copy SourceFiles="$(OutputPath)\$(XapFilename)" DestinationFiles="$(OutputPath)\$(OutputXapFilename)" />
</Target>