Since my first post of the idea in my article "Android mothership tablet for Android phone" back in June 21, 2010, there have been products like ASUS' PadFone/FonePad. However, that isn't exactly what I have in mind in terms of the integration.
My idea is more in the rim of software than hardware. However, the hardware integration can enhance the software implementation.
Now, here are some ideas for the software integration given that you got an Android phone and an Android Tablet with needed communications either through the WIFI or the USB :
1. Use the phone as the keyboard for the tablet.
2. Use the phone as the camera for the tablet.
3. Use the phone as the game control for the tablet.
4. Use the phone as the GPS for the tablet.
5. Share the storage between the phone and the tablet - you don't need an app on both device at the same time.
6. The display may support each other - like X-Window in the *nix world - this allows running apps on the tablet while display it on the phone or vice versa.
While I am writing, I just realize that what I really mean is providing a system level support that allows Android systems to support and share with each other the software and hardware resources.
As I mentioned earlier, using system buses may allow some sharing to be more intimate and efficient.