I ran into this same problem with Dell Latitude E6400. My problem was that I was trying to use the Intel XP driver; however, with WinPE 2.1 you need to use the Vista drivers. The fix was: I downloaded the vista driver for the Intel 82567 - extracted the files from the exe, and added them using the boot disk creator.

yup, thought of that and made sure i was using vista drivers. i found a work around injecting the drivers directly into the wim file using a peimg command. that made it work, from a quick googling, seems lots of people were having trouble getting the DC7900 to connect.

Working with a Dell E6400 Intel NIC, Bootwiz does not read the .INF file, because the window detailing the drivers you are about to load is blank. On servers where the driver was added with 6.5 SP1 installed and then upgraded to 6.9 SP1 the drivers are there in \Express\Bootwiz\paltforms\WinPE\x86\drivers, but trying to add them to a 6.9 SP1 install will only add the .INF and .TXT files to the .INF folder, the .DIN and .CAB are missing from the SYSTEM32 folder. This appears to be a bug in theBootwiz, as all these files are visible in the driver, and the driver loads correctly on a machine. I am using DS 6.9 SP1 (upgraded over Christmas break), WinPE 1.6 and XP.

If you are using DS 6.9 SP1, then make sure you install the maintence pack 1. I know there was a bug like this for adding drivers on WinPE PXE and the MP fixes this issue. This problem is only with 6.9 Sp1 as far as I know though.

When I downloaded a generic driver from Intel it reads the .INF file and populates the WinPE folders correctly, and works for imaging. So the problem is with the tweaking Dell did on the Intel driver. Thanks to everyone who responded with suggestions.

the MP1 did not do the trick. 
we've just used the vista nic drivers.