Windows Server 2008 install: wpeinit -unattend:unattend.xml does nothing

Thanks for the suggestions, redblacktree, but the XML file was in fact well formed, and it was working fine off a USB key.

I found out I could use /unattend with setup.exe, so I copied the entire Windows install DVD to a file server, booted the same WinPE image via PXE, then ran:

net use w: \\server\share
w:
cd x64
setup.exe /unattend:w:\unattend.xml

And it worked fine!

The other gotcha is that I had to run

wpeutil InitializeNetwork

before I could see the network.