OK, I can see a Mac Air with a flip-able keyboard being similar to the HP Slate in size/weight. I am not sure that it would EASY to do it with a spinable keyboard and still have the strength.
With the Slate, you can choose from a bunch of different keyboards. I chose one which matched the size of the unit pretty well, as you can see above.
I like the keyboard/case that is made for the iPad. It has an esiel in the back and the cover lies flat as a keyboard.
WKB-2000CB - Adesso Inc ::: Your Input Device Specialist ::: Keyboards | Mice | Touchpads | Tablets | Barcode Scanners | Mac Solutions | I/O Accessories
This looks like it is also compatible with the Slate. I may pick one up...