At the risk of showing how terrible I am at photoshop/paint, I made a mockup specifically showing off how ergonomics could possibly work with the detachable controllers:
Right half:
Note the trigger and shoulder buttons are on the "inside" of the controller, rather than the top. This makes it possible to hold a thinner control portion while still allowing easy access for the index finger. Also it could potentially allow for IR receivers on the top (a la Wiimotes) if they choose to include those.
Left half with split d-pad:
Attached to screen:
Obviously it cannot attach exactly at the shoulder/trigger but it can attach "on top" of those buttons, essentially sitting on top of the controller. This allows for easy support of the screen portion during normal gameplay (aka holding the device at a 0-45 degree angle from horizontal. It might need very strong connectors in order to hold it above your head (if you're playing with it in bed, for instance) but hey, this is a shitty mockup as advertised.
I really doubt it will look like this, but so far I haven't really seen anything outside of the simple rectangular control pieces that the Eurogamer sketch showed. They don't seem terribly comfortable to me.
Edit: It wasn't until I saw these pictures in dark theme that I realized how terrible I am at paint! Yikes!