Microsoft Build Task to Concatenate/Compress/Obfuscate javascript files.

build.xml

<?xml version="1.0" encoding="utf-8"?>

<Project DefaultTargets="Debug" xmlns="http://schemas.microsoft.com/developer/MsBuild/2003">
  <UsingTask TaskName="Concatenate" AssemblyFile="location to the dll that contains the concatenate class" />
  <ItemGroup>
    <FirstGroup Include="dir\javascript1.js" />
    <FirstGroup Include="dir\javascript2.js" />
    <FirstGroup Include="dir\javascript3.js" />
    
    <SecondGroup Include="dir\javascript4.js" />
    <SecondGroup Include="dir\javascript5.js" />
    <SecondGroup Include="dir\javascript6.js" />
  </ItemGroup>

  <Target Name="Debug">
    <Concatenate files="@(FirstGroup)" output="out\firstgroup.js" />
    <Concatenate files="@(SecondGroup)" output="out\secondgroup.js" />
  </Target>

  <Target Name="Release">
    <Concatenate files="@(FirstGroup)" output="out\firstgroup.js" IncludeHeaders="false" YUICompress="true" YUIObfuscateJavascript="true" />
    <Concatenate files="@(SecondGroup)" output="out\secondgroup.js" IncludeHeaders="false" YUICompress="true" YUIObfuscateJavascript="true" />
  </Target>

</Project>

Concatenate.cs

// Uses a port of the Yahoo YUI Compressor
// http://yuicompressor.codeplex.com/

using System;
using System.IO;
using System.Text;
using Microsoft.Build.Utilities;
using Microsoft.Build.Framework;
using Yahoo.Yui.Compressor;
using System.Resources;
using System.Collections.Generic;
using System.Reflection;
using System.Linq;
using System.Globalization;

public class Concatenate : Task {

    public bool YUICompress { get; set; }
    public bool YUIVerboseLogging { get; set; }
    public bool YUIObfuscateJavascript { get; set; }
    public bool YUIPreserveAllSemicolons { get; set; }
    public bool YUIDisableOptimizations { get; set; }
    public bool IncludeHeaders { get; set; }

    [Required]
    public string output { get; set; }

    [Required]
    public ITaskItem[] Files { get; set; }

    public Concatenate() {
        Files = new ITaskItem[0];
    }

    public override bool Execute() {
        FileInfo _fileOut = new FileInfo(output);

        string baseDir = new DirectoryInfo(".").FullName + "\\";

        if (Files.Length > 0) {

            string _header = "// {0} ";
            _header += Environment.NewLine;
            _header += "//==================================";
            _header += Environment.NewLine;
            _header += "{1}";
            _header += Environment.NewLine;
            _header += Environment.NewLine;

            if (!IncludeHeaders)
                _header = "{1}" + Environment.NewLine;

            StringBuilder sb = new StringBuilder();

            Log.LogMessage("Building ({0})", _fileOut.FullName.Replace(baseDir, ""));

            foreach (ITaskItem item in Files) {
                FileInfo FileIn = new FileInfo(item.ItemSpec);
                string data = string.Empty;

                try {
                    data = File.ReadAllText(FileIn.FullName);
                } catch {
                    Log.LogMessage("  => {0} CAN NOT READ, skipping", FileIn.FullName.Replace(baseDir, ""));
                }

                if (data.Length > 0) {
                    Log.LogMessage("  => {0}", FileIn.FullName.Replace(baseDir, ""));
                    sb.Append(string.Format(_header, FileIn.FullName, data));
                } else {
                    Log.LogWarning("  => {0} FILE IS EMPTY, skipping.", FileIn.FullName.Replace(baseDir, ""));
                }

                string nResult = sb.ToString();

                if (YUICompress == true) {
                    nResult = JavaScriptCompressor.Compress(
                      nResult,
                      YUIVerboseLogging,
                      YUIObfuscateJavascript,
                      YUIPreserveAllSemicolons,
                      YUIDisableOptimizations,
                      -1
                    );
                }

                try {
                    File.WriteAllText(_fileOut.FullName, nResult);
                } catch {
                    Log.LogWarning("Unable to write ({0}), path may not exist or file may be locked.", _fileOut.FullName);
                    return false;
                }
            }
            Log.LogMessage("{0} files processed.", Files.Length);
        } else {
            Log.LogWarning(
              "No file found to concatenate, Ignoring Request. \"{0}\" not created.",
              _fileOut.Name
            );
        }
        Log.LogMessage(Environment.NewLine);
        return true;
    }
}

This can be used for any text files that you wish to concatenate, not just js files. If you have a collection of css files that you would like to concatenate, exclude the YUICompress/YUIObfuscateJavascript attributes and the task will do the rest.