Amazon announced today their iPad compatible Kindle app. While it still needs to be approved by Apple, some competition in the e-book market would be a good thing. Barnes and Noble have similar plans to make books available on the iPad. Hopefully Apple doesn't lock things down to prevent competition with their own iBooks offering.
It will be also interesting to see what the reading experience is like on a backlit device such as the iPad vs. the e-ink display on the Kindle. I have used the Kindle app on my iPhone, and currently I prefer the e-ink display - although a lot of this has to do with screen size.