And as soon as that happens, Windows (Advanced) Firewall kicks into Public mode effectively locking your box down. Which is A Good Thing  – mostly, just not always what you want.

VMware KB article 1004813  also describes possible solutions to fix this. Aside from disabling the NICs (hey, we wanted to use those, remember?)  or changing the NICs over to Private Network (which you would have to do every reboot), you can configure the VMware adapters as so called Endpoint Devices per the procedure listed.
More information on the Endpoint device configuration is available on MSDN:
http://msdn.microsoft.com/en-us/library/bb201634.aspx
Reverting to the normal behaviour is as easy as flipping the DWORD to 0 or deleting the entry.

There is also a useful side effect to the *NdisDeviceType  setting:
Remote Desktop connections will keep working even with enabled virtual NICs, whereas by default Remote Desktop (TS) binds itself to all available network connections.

Which means I can finally RD into my desktop machine from my laptop with Host-Only and NAT adapters enabled to run a proper multi-machine, multi-network test.
Yay

[edit]

Link to KB article fixed 

Vista does not have this feature, but there is a workaround available. Installing the VHDMount utility from Virtual Server 2005R2 SP1 lets you use the VHD storage driver and the vhdmount utility.

1. Download the Virtual Server 2005 R2 SP1 setup from http://www.microsoft.com/windowsserversystem/virtualserver/downloads.aspx .
2. Run setup and choose Custom Setup type:

   
   

3. Verify the default selection for VHD Mount is marked:

   

   Click Next to install and finish the setup.

   

4. You can use the command line vhdmount utility to attach a .VHD file, or alternatively, use these registry entries to create a context menu for the four actions.

   Windows Registry Editor Version 5.00
    
   [HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Virtual.Machine.HD]
   @="Virtual Machine Hard Drive Image"
    
   [HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Virtual.Machine.HD\DefaultIcon]
   @="C:\\Program Files\\Microsoft Virtual PC\\Virtual PC.exe,-327"
    
   [HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Virtual.Machine.HD\shell]
   @="Mount"
    
   [HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Virtual.Machine.HD\shell\Commit]
    
   [HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Virtual.Machine.HD\shell\Commit\command]
   @="\"C:\\Program Files\\Microsoft Virtual Server\\Vhdmount\\vhdmount.exe\" /c \"%1\""
    
   [HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Virtual.Machine.HD\shell\Discard]
    
   [HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Virtual.Machine.HD\shell\Discard\command]
   @="\"C:\\Program Files\\Microsoft Virtual Server\\Vhdmount\\vhdmount.exe\" /d \"%1\""
    
   [HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Virtual.Machine.HD\shell\Dismount]
    
   [HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Virtual.Machine.HD\shell\Dismount\command]
   @="\"C:\\Program Files\\Microsoft Virtual Server\\Vhdmount\\vhdmount.exe\" /u /d \"%1\""
    
   [HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Virtual.Machine.HD\shell\Mount]
    
   [HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Virtual.Machine.HD\shell\Mount\command]
   @="\"C:\\Program Files\\Microsoft Virtual Server\\Vhdmount\\vhdmount.exe\" /p \"%1\""

   The result is a context menu on right-clicking a .VHD file like this.

   

   Mounting your disk file is now as easy as a right-click away, and the VHD will show up as a new disk in Explorer.