See www.aspcode.net for more info on this

Simple however:
  <UsingTask AssemblyFile="\code\tools\MSBuildTasks\GitVersion.dll"
                TaskName="GitVersion"/>


  <PropertyGroup Label="versioning">
    <versioningMajor>1</versioningMajor>
    <versioningMinor>0</versioningMinor>
    <versioningBuild>0</versioningBuild>
    <versioningRevision>0</versioningRevision>
    <LastCommit>not avail</LastCommit>
</PropertyGroup>	


  <GitVersion LocalPath="$(MSBuildProjectDirectory)">
    <Output TaskParameter="LastCommit" PropertyName="LastCommit" />
  </GitVersion>


Which you can then use like

  <AssemblyInfo CodeLanguage="CS"
      OutputFile="ReportUpdater\Properties\AssemblyInfo.cs"
      AssemblyTitle="ReportUpdater Last commit:$(LastCommit)"
      AssemblyDescription="ReportUpdater"
      AssemblyCompany="Systementor AB"
      AssemblyProduct="ReportUpdater"
      AssemblyCopyright="Copyright Systementor AB 2010"
      ComVisible="false"
      CLSCompliant="false"
      AssemblyVersion="$(versioningMajor).$(versioningMinor).$(versioningBuild).$(versioningRevision)"
      AssemblyFileVersion="$(versioningMajor).$(versioningMinor).$(versioningBuild).$(versioningRevision)"
/>