Entity Framework 4: How to find the primary key?

I am trying to create a generic method using EF4 to find the primary key of an object.

example

public string GetPrimaryKey<T>()
{
    ...
}

To give more info I am working off of the Tekpub StarterKit and below is the class I am trying to get up and running

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Data.Objects;
using System.Data.Objects.ELinq;
using System.Data.Linq;
using Web.Infrastructure.Storage.EF4;

namespace Web.Infrastructure.Storage {
public class EFSession:ISession {
    PuzzleEntities _db;//This is an ObjectContext
    public EFSession() {
        _db = new PuzzleEntities();
    }

    public void CommitChanges() {
        _db.SaveChanges();
    }
    /// <summary>
    /// Gets the table provided by the type T and returns for querying
    /// </summary>
    private ObjectSet<T> GetObjectSet<T>() where T:class {
        return _db.CreateObjectSet<T>();
    }

    private T GetByPrimaryKey<T>() where T: class
    {
        .....
    }

    public void Delete<T>(System.Linq.Expressions.Expression<Func<T, bool>> expression) where T: class{

        foreach (T item in All<T>().Where(expression))
        {
            GetObjectSet<T>().DeleteObject(item);
        }
    }

    public void Delete<T>(T item) where T : class {
        GetObjectSet<T>().DeleteObject(item);
    }

    public void DeleteAll<T>() where T : class {
        foreach(T item in All<T>())
        {
            GetObjectSet<T>().DeleteObject(item);
        }
    }

    public void Dispose() {
        _db.Dispose();
    }

    public T Single<T>(System.Linq.Expressions.Expression<Func<T, bool>> expression) where T:class {
        return GetObjectSet<T>().SingleOrDefault(expression);
    }

    public IQueryable<T> All<T>() where T : class {