The PlayStation Store will offer two separate downloads: one for 2D, and one for 3D. Both demos include the same "Icy Incursion" stage, but the 2D version features a split-screen co-op mode not available in the 3D version. If you download the 3D version but own a 2D display, the blog states it will still play just fine, after giving a message that it doesn't detect a 3D TV. Both demos will also feature Move support.
If you're a Plus subscriber, you may not have to lift a finger to snag the demo. The 2D version will be included in the Automatic Download feature, so if you have the option enabled you'll wake up to find Helghast already hanging out on your PS3. But like all uninvited houseguests, they'll probably just drink all your beer and stain the couch.