It should be legal. It's your device, and you should be able to do whatever the hell you want to it. Having said that, Sony doesn't have to allow new games or online services to work with modified PS3s. I'm sure that's how they'll fight back. But the thing is that this isn't even a permanent mod. It's a USB dongle, which can be plugged in or pulled off at any time. Jekyll or Hyde in an instant. New PS3 firmware is the only way to counteract that, and my guess is that this company will issue firmware revisions to its USB device to match (if they can). Same cat and mouse game as the PSP.
I'm assuming this thing is real, which so far is not really established.