http://aspnetwebstack.codeplex.com/workitem/648

PatchKeyMode.cs

using System;
using System.Net.Http;
using System.Net.Http.Headers;
using System.Web.Http;
using System.Web.Http.OData;
using System.Web.Http.OData.Builder;
using System.Web.Http.OData.Formatter;
namespace Application
{
    public class Program
    {
        public static void Main()
        {
            HttpConfiguration config = new HttpConfiguration();

            ODataConventionModelBuilder builder = new ODataConventionModelBuilder();
            builder.EntitySet<Person>("People");

            ODataMediaTypeFormatter formatter = new ODataMediaTypeFormatter(builder.GetEdmModel());
            formatter.PatchKeyMode = PatchKeyMode.Patch;

            config.Formatters.Insert(0, formatter);

            config.Routes.MapHttpRoute("default", "{controller}");
            config.Routes.MapHttpRoute(ODataRouteNames.GetById, "{controller}({id})");
            config.Routes.MapHttpRoute(ODataRouteNames.Default, "{controller}");

            HttpServer server = new HttpServer(config);

            HttpClient client = new HttpClient(server);

            HttpRequestMessage request = new HttpRequestMessage(HttpMethod.Put, "http://localhost/People");
            request.Content = new StringContent("{ 'PersonId': 'b269c49f-8a90-41d6-b102-7cfba3812b1c', 'FirstName': 'sample string 2', 'LastName': 'sample string 3', 'IsActive': true, 'NumVacationDays': 5, 'Salary': 6.1    }");
            request.Content.Headers.ContentType = MediaTypeHeaderValue.Parse("application/json;odata=verbose");
            Console.WriteLine(client.SendAsync(request).Result.Content.ReadAsStringAsync().Result);
        }
    }

    public class PeopleController : ApiController
    {
        public Person Put(Delta<Person> person)
        {
            var p = person.GetEntity();

            Person existingPerson = new Person { PersonId = p.PersonId };

            person.Patch(existingPerson);

            return existingPerson;
        }
    }

    public class Person
    {
        public Guid PersonId { get; set; }

        public string FirstName { get; set; }

        public string LastName { get; set; }

        public bool IsActive { get; set; }

        public int NumVacationDays { get; set; }

        public double Salary { get; set; }
    }
}

The output on my box is,

{
  "d": {
    "__metadata": {
      "id": "http://localhost/People(guid'b269c49f-8a90-41d6-b102-7cfba3812b1c')",
      "uri": "http://localhost/People(guid'b269c49f-8a90-41d6-b102-7cfba3812b1c')",
      "type": "Application.Person"
    },
    "PersonId": "b269c49f-8a90-41d6-b102-7cfba3812b1c",
    "FirstName": "sample stri ng 2",
    "LastName": "sample string 3",
    "IsActive": true,
    "NumVacationDays": 5,
    "Salary": 6.1
  }
}