Impulse buying is hugely enabled when you own an ereader. Pushing a button to purchase a "weightless" book that takes up no room in your house, that you can hide from your spouse/parent, etc.
- takes away the "guilt" or logic of asking yourself - "do I really want another book?"
This buying is confined to ereaders - which probably explains the (at least initial) lack of impact on pbook sales.
I remember reading an interview with Colin Powell in the NYT Book Review - where he admitted he has "hundreds" of books on his Kindle that he doesn't even remember buying.