Sadly I very much doubt they have populated any of that circuitry or even routed those pins to any part of the board for future expansion. It's not a feature that Dell (or HTC for that matter) would consider important and the kind of PCB level real estate required, given the size limitations of the device, really don't justify the benefits.

Given that the PXA270 is a BGA device common practice is to route signals out from the chip across multiple layers of PCB through vias and, once again going back to fact that it is valuable real estate that could be better used for something more important, I doubt that those pins are even connected to the board in a way that would allow you access.

I would like to be wrong about this, as it would be nice to use USB without buying a Ratoc card, but I work with PCB designers and know just how difficult it can be to justify routing every pin of a device. Just leaving four pins completely disconnected can make your life thousands of times easier.