I think a DAC option for the BDP-1 would be desirable to many and I think it would be smart option to offer if there's room in the box.
For me, I would rather have an internal HDD bay, so I don't have to have a USB HDD out in the open and deal with another wall wart. Of course, I'm not afraid to swap internal HDDs or even use the USB enclosures like Thermaltake's:(
http://www.newegg.com/Product/Product.aspx?Item=N82E16817153066&cm_re=usb_hdd_enclosure-_-17-153-066-_-Product)
Since both the 2.5" and 3.5" drives now conform to mechanical connection standard you could make this real simple such that it can slide in from the back or a front slot and which could lock by shutting the door. This would be no different than enterprise server hardware HDD loading mechanism (Also used by the Mac Pro and Drobo External Drives). Potentially this would mostly be a mechanical change to the existing design. You could easily use a SATA->USB adapter to keep everything simple internally and stick to 2.5" drives that can be USB powered. Ideally you would want to use a 2.5" SSDs to minimize vibration and noise from the traditional HDDs.
This would just augment the front and rear USB ports along with the support for the NAS. And then I can switch between a thumbdrive, NAS, internal HDD or perhaps if my music collection gets truely out of control the USB drive with the wall wart.
Sadly though, I realize that my desire would be intimidating to the average user even though I think it would be rather easy to offer.
Jim