recently I filed this bug for banshee:
http://bugzilla.gnome.org/show_bug.cgi?id=327595
This one was closed with a comment that there could be a problem with dbus and mono.

I've done some debugging and it seems, that this could be true ;-)

in dbus-0.60/mono/Connection.cs:
    internal static Connection Wrap(IntPtr rawConnection) 
    {
      if (slot > -1) {
	// Maybe we already have a Connection object associated with
	// this rawConnection then return it
	IntPtr rawThis = dbus_connection_get_data (rawConnection, slot);
	if (rawThis != IntPtr.Zero) {
	  return (DBus.Connection) ((GCHandle)rawThis).Target;
	}
      }
      

It seems, that as long dbus_connection_get_data returns a value != NULL, it is
assumed that there is still a Target associated to the GCHandle object.

But the GCHandle's Target is associated as Weak target:
	      rawThis = GCHandle.Alloc (this, GCHandleType.WeakTrackResurrection);

I'm not very familiar with mono, but as far as I read this means, that it is
possible that the Target of the GCHandle can be freed by the garbage collector
(otherwise GCHanldeType.Normal should be used).

This means, that even if there is still a GCHandle, it is possible that the
target is already cleaned up. In this case, a null value is returned which
results later in an exception.

I've tested two possibilities to solve the problem:

1. associate the target as GCHandleType.Normal

2. only assume that there is already an DBus.Connection if the GCHandle's Target
has the type DBus.Connection:
...
	if (rawThis != IntPtr.Zero && ((GCHandle)rawThis).Target ==
typeof(DBus.Connection)) {
...

Both of them work.

It would be good if somebody could have a look at this issue.

Thank you very much!