HelloWorld.cs

using System;
using Microsoft.Build.Framework;
using Microsoft.Build.Utilities;

namespace MSBuildHelloWorldTask
{
    public class HelloWorld : Task
    {
        [Required]
        public string FirstName
        { get; set; }

        public string LastName
        { get; set; }

        [Output]
        public string Message
        { get; set; }

        public override bool Execute()
        {
            Message = string.Format("Fullname: {0} {1}", FirstName, LastName);
            Log.LogMessageFromText(string.Format("Hello {0} {1}!", FirstName, LastName), MessageImportance.High);
            return true;
        }

    }
}

HelloWorld.lproj

<Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003" ToolsVersion="4.0" DefaultTargets="Demo">

  <UsingTask TaskName="HelloWorld" 
             TaskFactory="CodeTaskFactory" 
             AssemblyFile="$(MSBuildToolsPath)\Microsoft.Build.Tasks.v4.0.dll">
    <ParameterGroup>
      <FirstName ParameterType="System.String" Required="true" />
      <LastName  ParameterType="System.String" />
    </ParameterGroup>
    <Task>
      <Reference Include="Microsoft.Build.Framework" />
      <Reference Include="Microsoft.Build.Utilities.v4.0" />
      <Code Source="HelloWorld.cs" Language="cs"  />
    </Task>
  </UsingTask>
  
  <Target Name="Demo">
	  <HelloWorld FirstName="Jonny" LastName="Yu" />
  </Target>
</Project>

This is documented behavior at:
http://msdn.microsoft.com/en-us/library/dd722601.aspx