MacTab by Yann Le Corroler
Simplicity is king in Yann Le Corroler's MacTab. All it takes is a thin backing frame to hold the computer upright, and I'm assuming the ultrathin keyboard can be attached for maximum portability.
MacTouch by TechnoMinds
The TechnoMinds at DeviantArt have envisioned a concept that will surely induce mass carpal tunnel syndrome (if that hasn't already happened). I imagine this computer will sit at the edge of your desk, providing a nice amount of neck strain, too.
LMac Touch by Mayyen Chan
If tablets take off, the battle that follows will no doubt be over who can make the best stand. Mayyen Chen's attempt combines a picture frame stand and a hinged platform for adjusting the angle. There's a wheel for moving the whole thing around, so I guess the tablet isn't very lightweight.
Apple Tablet by MacLife
Admit it, you've seen this one everywhere. That was MacLife, in one of the site's renderings. Of course, this is exactly what the real Apple tablet will look like, size, interface, and all. On this, there can be no debate.