
Google has continued to maintain that smartphone usage is entirely complementary to PC usage and is not cannibalizing it. However studies by Google and now Nielsen are showing something different when it comes to tablets -- they're being substituted for PCs in a variety of cases.
Nielsen conducted a Q1 survey of tablet owners and found the following:
Here's a breakdown of the the impact of tablet ownership on usage of other devices:


Here's data from Google also showing reduced PC usage after buying a tablet:

Source: Google/AdMob, March, 2011 (n=1,403)