Called XPS Tech support to find the part number for the interposer (the thing that connects the hard drive to the laptop). The support guys couldn't find the part, sent me to the parts department. Disconnected.
Called the parts department. The parts guy couldn't find the part (actually he told me they didn't have any in stock, but when I asked for the part number he was looking up he admitted he couldn't find the part at all). He told me to talk to tech support. Transferred. Disconnected.
Called the number he gave me to call, ended up with Inspiron tech support, which told me they couldn't be of much use. Gave me a case number this time. Got to XPS support again, the guy says he knows what part I'm talking about, needs some time to find it. While waiting I found a picture of what I think it looks like. Showed it to the tech guy, who thinks it is a generic part compatible with many different models. I happen to have a Latitude D800 that has the exact same part, I yank it out and plug it into the XPS, works perfectly. Ordered 2 from Ebay for $4 each with $4 shipping. Now I just need some screws...
Hope this helps anyone else looking for the piece. I don't know the Dell part number, but 0338 is printed on the piece, which is made by FoxConn.