I realize it has been ages since the previous post in this thread. Still I like to add some info.
Other than (u)SD, CF and SM cards, xD cards do not have an internal IDE-adapter. The adapter is necessary to translate between "flashy" memory (MTD, memory technology device) and block device memory.
xD cards omit the adapter which could lower the cost of the card. The adapter has to be implemented either in hardware or in software. Some cameras have hardware for that, some leave it to the host: the (MS Windows) driver that came with the camera.
Linux comes with MTD-drivers. Not important for this thread but maybe handy to know is that the implementation was not optimal, resulting in slower access to the MTD.
Important for this story though, is that for quite a while cheaper "all-in-one" card readers would not (fully/reliably) support xD, because the necessary hardware would be missing.
A lot of info can be found at http://www.linux-mtd.infradead.org/faq/general.html
and on wikipedia under MTD and FTL (flash translation layer)
Sorry for not deeplinking to the appropriate information, it has been a while since I first took that info to me and the details are a bit vague.