publicizes user stuff, changes the IUser.Id to an int - no need for it to be an object and casted everywhere. Updates the login logic to perform the mapping logic for custom membership providers with an extension method.

src/Umbraco.Core/Models/Membership/IProfile.cs

@@ -3,6 +3,10 @@
     /// <summary>
     /// Defines the the Profile interface
     /// </summary>
+    /// <remarks>
+    /// This interface is pretty useless but has been exposed publicly from 6.x so we're stuck with it. It would make more sense 
+    /// if the Id was an int but since it's not people have to cast it to int all of the time!
+    /// </remarks>
     public interface IProfile
     {
         object Id { get; set; }

src/Umbraco.Core/Models/Membership/IUser.cs

@@ -9,7 +9,7 @@ namespace Umbraco.Core.Models.Membership
     /// <remarks>Will be left internal until a proper Membership implementation is part of the roadmap</remarks>
     public interface IUser : IMembershipUser, IProfile
     {
-        new object Id { get; set; }
+        new int Id { get; set; }
 
         int SessionTimeout { get; set; }
         int StartContentId { get; set; }

src/Umbraco.Core/Models/Membership/User.cs

@@ -19,7 +19,7 @@ namespace Umbraco.Core.Models.Membership
     /// </remarks>
     [Serializable]
     [DataContract(IsReference = true)]
-    internal class User : TracksChangesEntityBase, IUser
+    public class User : TracksChangesEntityBase, IUser
     {
         public User(IUserType userType)
         {
@@ -45,7 +45,7 @@ namespace Umbraco.Core.Models.Membership
 
         private readonly IUserType _userType;
         private bool _hasIdentity;
-        private object _id;
+        private int _id;
         private string _name;
         private Type _userTypeKey;
         private readonly List<string> _addedSections;
@@ -242,6 +242,12 @@ namespace Umbraco.Core.Models.Membership
 
         #region Implementation of IProfile
 
+        object IProfile.Id
+        {
+            get { return Id; }
+            set { Id = (int)value; }
+        }
+
         [DataMember]
         public string Name
         {
@@ -355,7 +361,7 @@ namespace Umbraco.Core.Models.Membership
         }
 
         [DataMember]
-        public object Id
+        public int Id
         {
             get { return _id; }
             set