Although brands like Westen Digital do spin down their drives but remain connected when idle and so would not need this feature, it's the Buffalo Linkstation which would benefit most. It's power saving option when in 'Auto' mode is to enter standby if it doesn't receive a wake on LAN magic packet every 360 seconds. The reason for this is presumably because it was designed with PC use in mind and the proprietary Buffalo PC software runs in the background and repeatedly sends the magic packet as long as it is running.
What I hope to be able to achieve is accessing DLNA content from the server on my iPhone or iPad without needing the PC to be turned on too. Currently I can use a separate iPhone app (SleepOver) to wake up the NAS but it shuts down again after a couple of minutes. If the PlugPlayer app had an option to send a magic packet every couple of minutes this would solve the problem.
I recognise that this is quite a niche feature as it only seems to be necessary for Buffalo NAS drives but they're quite popular and it may be useful for other brands too. The way I would implement it is to have an option similar to the SSDP re-advertisment switch in the device settings menu. Perhaps 'Keep awake with WOL' - if switched to On, the user would be presented with two further text boxes, one for the device MAC address and the other for the interval between which magic packets should be sent. Perhaps there could also be an option to automatically send the magic packet as soon as PlugPlayer starts.