Not right out of the box you can't.
seem to remember that someone successfully networked a 721 using the USB port. In theory, you could then use the 721 for remote storage, but who knows what kind of troubles it would cause
Has anyone documented what the keystrokes do? I'm experimenting with an el-cheapo "Micro Innovations" wireless keyboard/mouse combo connected to the usb port, some of the keys seem to correspond to remote buttons, it works good for the search feature and the mouse appears to work only in the guide. One downside I've noticed is that the 721 seems to get real sluggish and has spontaneously rebooted a couple of times.