You can already do that with the SanDisk Media Drive and stream wirelessly (via WiFi) from the drive to your smartphone, iPad, etc.
http://www.sandisk.com/products/wireless/media-drive/
I have a 32GB one and it also has an SD card slot for even more storage. No, it doesn’t work with any of SanDisk’s dedicated mp3 players, but with more and more people having smartphones already it’s a way to have a huge database of music and/or movies at your disposal withour any cords getting in the way. 
