As the owner of a mid-2010 Macbook Pro and a first-gen iPad, I think the new Macbook Air, which is a significant step in that direction, could replace both devices for me.
The iPad cannot be used for productivity work (coding, writing, even longer emails), but my 15" MBP is relatively clunky, especially for the type of consumption activities that the iPad excels at (i.e. reading on the couch, watching a TV episode, etc.) I wish I could have the best of both worlds.
The new iPad version of Garage Band really captures this: in some ways it's better than the Logic Pro I have on my laptop; I can play keys on it, I can trigger drums with my fingers, etc., but it's painfully underpowered (in terms of CPU, memory, I/O, etc.) to comfortably record a serious track on it (despite what Gorillaz accomplished).
I'm waiting for a Macbook Air with a full touch screen, and a new OS that fully combines the simplicity and interactivity of iOS with the power and breadth of OSX.