Nielsen has done some research on tablet (e.g., iPad), electronic reader (e.g., Kindle) and smartphone usage. No big surprises here -- people read in bed and a Kindle doesn't generate its own light (so is less likely to distract someone with whom you might share said bed) and there are already a number of TV guides that work on the big screen of a tablet (none of them work on a Kindle or smartphone).
Smartphone tops for "With family/friends", "Waiting for something", "Shopping/running errands" and "Commuting" -- the most mobile and social activities.
Reader tops for "Lying in bed" and "Other activities" (like, er, reading?)
Tablet tops for "Watching TV" (multitasking or using these guides, I would imagine), "Attending a Meeting/Class" (taking notes or pretending to do so) and "In the Bathroom" (well, it is about the size of a magazine).
In the U.S., Tablets are TV Buddies while eReaders Make Great Bedfellows (Nielsen Wire)