What's interesting is that they have only sold 1.5M books, or 1.5 books per device. Amazon Kindle should be safe.
I have an iPad but haven't bought any books, but I don't have a Kindle either. I imagine there are people who are buying iPads instead of Kindles, and they are responsible for a disproportionate number of the 1.5M books.
Also, I've read a lot of comments on how it is hard to read on an iPad (and some in the other direction) but for what it's worth, I'm in the latter camp. I've done a lot of reading (hurray Instapaper!) on my iPad and it has been a very good experience.