I just recently got a kindle, but I was reading books on my iphone/PC waaaaaaay before that purchase. I've only ran into one book that I purchased so far that won't let me have the book on multiple devices at the same time (as in, it's on my iphone currently, and I'd have to delete it to read it on a diff device).
Ran across the free kindle app earlier today and downloaded it. I had no idea that Amazon offered that. Good Deal !!!
I have the Kindle app on my Samsung Galaxy Tab 2 tablet. I also have the Nook. It's great turning my tablet into an E-reader device and I'm not stuck with one format or the other.