Amazon has released a Kindle application for the iPhone. It nicely synchronizes with your Kindle library (which is all stored "in the cloud") but, strangely, it seems to only pick up books and not periodicals or blogs.
The display is okay (I mean, it's the iPhone display. Duh.), nothing compared to the Kindle's eInk. The big win, though, is that the iPhone supports color, so this might make the iPhone good for reading Kindle content with images.
It appears that there's no facility for taking notes or highlighting, which is a fairly big deal for technical readers (on the other hand, since it's the iPhone I can flip over to Evernote, take a note there, and it synchs up with all of my Evernote databases). (P.S. Evernote is no OneNote, but there is no OneNote for the Mac, which is a huge disappointment.)