1. Download MSBuild Community Task
2. Add the following to the .csproj file of the project you want to follow the svn build numbering. Add just before </Project>

  <!-- Import of the MSBuildCommunityTask targets -->
<import Project="$(MSBuildExtensionsPath)\MSBuildCommunityTasks\MSBuild.Community.Tasks.Targets" />

<!-- to manage version number -->
<target Name="Version">
  <version VersionFile="version.txt" RevisionType="Increment">
    <output TaskParameter="Major" PropertyName="Major" />
    <output TaskParameter="Minor" PropertyName="Minor" />
    <output TaskParameter="Build" PropertyName="Build" />
    <output TaskParameter="Revision" PropertyName="Revision" />
  </version>
</target>

  <!-- to generate our personnal version info -->
  <target Name="AssemblyInfo">

  <svnversion LocalPath="$(MSBuildProjectDirectory)" Condition="Exists('$(ProgramFiles)\subversion\bin\svn.exe')">
    <output TaskParameter="Revision" PropertyName="Build" />
  </svnversion> 

  <assemblyinfo CodeLanguage="CS"
    OutputFile="Properties\AssemblyInfo.cs"
    AssemblyTitle="WindScanner"
    AssemblyDescription=""
    AssemblyCompany="Risø"
    AssemblyProduct="windscanner"
    AssemblyCopyright="Copyright © 2010"
    ComVisible="false"
    Guid="88812638-9547-4480-3bf4-4fe25103b35c"
    AssemblyVersion="$(Major).$(Minor).$(Build).$(Revision)"
    AssemblyFileVersion="$(Major).$(Minor).$(Build).$(Revision)"
    Condition="$(Revision) != '0' "/>
</target>

<!-- We launch these two targets -->
<target Name="BeforeBuild">
    <calltarget Targets="Version" />
    <calltarget Targets="AssemblyInfo" />
</target>

<target Name="AfterBuild">
</target>

Change the path of your svn installation if necessary.

You can now use stuff like

public static String Version
        {
            get
            {
                return System.Reflection.Assembly.GetExecutingAssembly().GetName().Version.ToString();
            }
        }

And the the changes reflecting the svn build!