After nearly a hour of searching for an annoying bug (which turned out to be a new statement one line outside the loop it should have been in)....Multi-Regions in a single instance work, there is no Grid mode communication, so it can only be tested in sandbox mode right now.

All you need to do is put .xml files for each region you want to be ran by the single server inside the "bin\Regions" folder (make sure none of the settings in those .xml files conflict with each other) and then startup the server and login, Any neighbouring regions that are being ran in the instance should show up.  (However there is still no movement or anything) 
Now time to refine the communications interfaces!

1 files changed, 13 insertions, 7 deletions

diff --git a/OpenSim/OpenSim.World/World.cs b/OpenSim/OpenSim.World/World.cs
index 2c5971d..1d56ea3 100644
--- a/OpenSim/OpenSim.World/World.cs
+++ b/OpenSim/OpenSim.World/World.cs
@@ -504,16 +504,22 @@ namespace OpenSim.world
         /// </summary>

         protected void InformClientOfNeighbours(IClientAPI remoteClient)

         {

+           // Console.WriteLine("informing client of neighbouring regions");

             List<RegionInfo> neighbours = this.commsManager.RequestNeighbours(this.m_regInfo);

 

-

-            for (int i = 0; i < neighbours.Count; i++)

+            //Console.WriteLine("we have " + neighbours.Count + " neighbouring regions");

+            if (neighbours != null)

             {

-                AgentCircuitData agent = remoteClient.RequestClientInfo();

-                agent.BaseFolder = LLUUID.Zero;

-                agent.InventoryFolder = LLUUID.Zero;

-                agent.startpos = new LLVector3(128, 128, 70);

-                this.commsManager.InformNeighbourOfChildAgent(neighbours[i].RegionHandle, agent);

+                for (int i = 0; i < neighbours.Count; i++)

+                {

+                   // Console.WriteLine("sending neighbours data");

+                    AgentCircuitData agent = remoteClient.RequestClientInfo();

+                    agent.BaseFolder = LLUUID.Zero;

+                    agent.InventoryFolder = LLUUID.Zero;

+                    agent.startpos = new LLVector3(128, 128, 70);

+                    this.commsManager.InformNeighbourOfChildAgent(neighbours[i].RegionHandle, agent);

+                    remoteClient.InformClientOfNeighbour(neighbours[i].RegionHandle, System.Net.IPAddress.Parse(neighbours[i].IPListenAddr), (ushort)neighbours[i].IPListenPort);

+                }

             }

         }