If it's download-only, i could equally see it being PS360 exclusive, if only due to WiiWare's low, low file ceiling, but i don't think it will be downloadable. SEGA's going to want to go for the throat with this
I agree that HD is probably just being used as a buzzword by the marketers here, that it doesn't necessarily imply HD exclusive.
Either way, the way they're choosing to promote it has put a bad taste in my mouth, as a loyal fan of 3D Sonic and as a Nintendo owner, but i'll wait to see something tangible before i pass any real judgment.