The Kindle for iPhone/iPod Touch application comes with Whisper Sync. If you have a Kindle device, it will sync up your purchases with it and vice versa. It will also sync your place so you know where you left off, no matter which device you use.
This is the Home screen, where your purchased books are listed. If you're a previous Kindle owner, you will see an Archived Items folder with a list of your previous purchases. Select the ones you want on the iPhone, and the iPhone will download them.
There is a huge potential for comic books in the Kindle. The device's e-ink only supports black and white for now, but seeing the color version on the iPhone application shows the potential it has for supporting comics, graphic novels, and illustrations.