We can't say for sure if the Apple iPad is the cause, but it's a pretty good bet the iPad and the iBook app are part of the problem for Amazon and Barnes and Noble. Both booksellers today dramatically cut the price of their competing e-book readers.
Nook started the ball rolling downhill this morning when the original Nook, priced at US$259, was cut to $199, and a new Wi-Fi only model was introduced at $149. Now Amazon has just responded, cutting the $259 Kindle to $189. Whew! It's getting competitive out there.
Many people still prefer a dedicated book reader, but the iPad has come on strong. I've read several books on my Kindle, and the screen is detailed, but I actually prefer reading on the iPad where I don't have to worry about finding an external light source. I also prefer the color illustrations on the iPad for the books that feature them.
It will be interesting to see how this all winds up. Of course you can read Kindle books and purchases from the Nook store on your iPad or iPhone (and on your Mac), so readers have plenty of choices. Sadly, you have to juggle a lot of different e-reader apps on each device, but we all benefit as consumers when the competition gets up and running.