The end result is that the capability is there -- the iPod uses the standard IEEE1394 specification for transferring files to and from the hard drive, AFAIK. If you plug the iPod into a Linux box that supports FireWire hard drives, I'd be willing to bet it would show up as a regular hard drive. That means that companies are free to develop peripherals to the extent that IEEE1394 supports - read and write, which is all a cell phone user would need for movie-playing ability.
The only reason Apple would need to get involved is if a company wanted to make use of the UI or somehow modify the UI of the iPod to suit their peripheral, or tap into some of the extended software of the iPod. Apple doesn't market the iPod as a Jack-of-all-trades device, and as such, shouldn't be sanctioning companies left and right to make peripherals outside of music devices to extend the functionality of the iPod. I'd be willing to bet Belkin made their iPod card reader peripheral without help or intervention from Apple (sure, they probably got the go-ahead from Apple, but that's not helping). And as such, it's pretty much up to the 3rd party whether or not they include the ability to use the iPod as a hard drive with their peripherals. How could Apple help them? Point them to a document on the IEEE1394 standard, which they don't manage?