There were a couple of tricks to getting Aero 3D working in my Virtualbox VM:

First, I had to make sure the Aero theme was disabled in Windows before enabling 3D graphics in the virtual machine settings.  Here’s how:

After switching to the Windows Basic theme, shut down.  Then, enabled 3D in the VM settings.  Started back up, enabled an Aero theme, and it looked beautiful — except for IE9, which in my case crashed mercilessly.

IE9 has an Accelerated Graphics option in Internet Options.  Switched it to software-only acceleration, and IE9 was happy.  The End.