Mixed Platform (C# C++) COMException 80040154 on Vista 64 SP1?

I've been banging my head on this for a week now. Everything worked
fine on Vista 32-bit. Yet no fun allowed under Vista SP1 64-bit.

I thought it was a x64, VC90 CRT issue, or a Vista SP1 issue. I think
now it is not a CRT issue though SP1 or 64-bit are still suspects.

* My ATL/COM component registers fine at the command line using regsvr32.
* My solution is configured for "Mixed Platforms".
* It is 90% C# except for one C++ ATL/COM component that interfaces to
the setupapi to enumerate and identify USB / serial ports.
* I disabled UAC.
* I am using Visual Studio 2008
* In case of some VC90 CRT issue, I changed the COM component to use
static MFC and ATL.

Attempting to create a COM object from my C# app throws the below
COMException:

DeviceHelper.CSerialPortClass serialPort = new
DeviceHelper.CSerialPortClass();

"Retrieving the COM class factory for component with CLSID
{CD246540-9C6B-4FE1-95B5-DA3D20C3C308} failed due to the following
error: 80040154."} System.Exception
{System.Runtime.InteropServices.COMException}

Any tips or suggestions to help debug this are much appreciated.

-Ed