Apparently if you are having all sorts of DNS/networking/DHCP issues with these demo VHDs, the product group says that it is very important that you IMPORT the VMC settings and don't create your own VMC settings files (using the IW Demo VHDs).
That is, read the directions on the MS Download Center page (and follow them :-)).
I'll be trying the import as soon as I recopy the 47GB of VHD files.
Update 1: You need to use Windows Server 2008 *R2* Hyper-V or the config import won't work (i.e. with plain old Windows Server 2008 Hyper-V, the import will fail).
Update 2: Bryan recommended installing Windows Server 2008 R2 as an upgrade. The upgrade displays a screen to warn you about potential upgrade changes. One of them is related to Hyper-V. Checkout MS KB Article 957256. It took a good part of the evening (measured in hours). Hyper-V imported the 2 VMs with no problem.