Hi,
I just joined to tag into this thread. I'm having the same issue, except, it seems to occur in waves.
I'll have the Server sleeping for days at a time and only waking when I need to use it (ie. I access the network drive). It'll go back to sleep once inactive and no funny behaviour.
But then, on some weird random chance, it'll wake up on its own. When it does, I get the exact same behaviour as Pooky, where even if I try putting it back to sleep, it stays for about 30 seconds and then comes right back up again. Much like Pooky, I have verified and isolated any other variables and can advise that it is the router that's sending a signal (as proved a couple of posts ago).
I've sometimes ignored it and after a day or so, the odd behaviour stops but it tends to re-occur every so often. I'm going to try setting the network to "Public" as the previous poster suggested, but I'm somewhat skeptical of that solution as it doesn't seem to make any sense. The alternative, of course, as mentioned is to set WOL to only occur with Magic Packets. But that tends to defeat the purpose with devices that can't send Magic Packets (I'm crossing my fingers that the squeezebox Duet I ordered can)
Anyways, my apologies for the long post, will report back tonight.