Looks like Apple's rumored tablet may employ an "articulating frame" surface. This simply means that when in keyboard mode, the surface of the tablet will be able to create bumps to give a true tactile experience. This technology is detailed in an application titled "Keystroke Tactility Arrangement on a Smooth Touch Surface."
Excerpt from the filed application:
"The articulating frame may provide key edge ridges that define the boundaries of the key regions or may provide tactile feedback mechanisms within the key regions. The articulating frame may also be configured to cause concave depressions similar to mechanical key caps in the surface."
The invention filed by Apple on Aug. 27, 2009, is credited to Steven P. Hotelling, Christoph H. Krah and Brian Quentin Huppi of California.