This guide only works on WS 2008 or Vista. The WAIK is supported on WS 2003 and XP too, but the driver injection will fail.
You can download the WAIK here.
Install the WAIK, open the Windows PE Tools Command Prompt. Also, have the WS 2008 boot.wim ready, and mount the VMWare Tools image by clicking "Install VMWare Tools", this will mount the drivers on the cd-rom drive.
If you are not doing these steps on a virtual machine, you can also just download these drivers (vmxnet).
1. copype x86 c:\windowspe
2. imagex /info "c:\path\to\x86\boot.wim"
In the WIM Information, look for the Boot Index, in this case the boot index is 2. We will use this in the next step.
3. imagex /mountrw "c:\path\to\x86\boot.wim" 2 c:\windowspe\mount
4. peimg /inf="c:\path\to\vmxnet\win2k\vmxnet.inf" c:\windowspe\mount\windows
5. peimg /inf="c:\path\to\vmxnet\win2k\vmware-nic.inf" c:\windowspe\mount\windows
You should see "PEIMG completed the operation successfully". If you would do this on a XP or 2003 machine, the message would be "PEIMG failed to complete the operation with status code 0x800703fb".
It's possible that this WILL work in future versions, but at the moment it doesn't
6. imagex /unmount c:\windowspe\mount /commit
The boot.wim image has now been injected with the VMWare NIC drivers.
There is an alternative to this, instead of doing the above driver injection procedure, you can add this to your .vmx file to have network support too: ethernet0.VirtualDev="e1000"