[JakeS]

Here's how PockeTwit launches URLs (in C#):

  string URL = "http://code.google.com/p/pocketwit/";

  System.Diagnostics.ProcessStartInfo pi = new System.Diagnostics.ProcessStartInfo();
  pi.FileName = URL;
  pi.UseShellExecute = true;
  System.Diagnostics.Process p = System.Diagnostics.Process.Start(pi);

So you can see it's just telling the OS to launch the URL. I hope this will help somehow. I'm sorry I don't know what registry settings the OS uses to determine how to handle it.