PhotoFlip! (free) is a really clever idea that lets you add notes to any photo. By just creating a shell around your existing photos with metadata, it doesn't re-copy the photos wasting space on your iOS device. Notes can be at the top or bottom of the photo, and photos can be shared with or without notes through the app via text messaging or email. The notes can be synched to the identical photos on your other iOS devices through the cloud.
The downsides are the app doesn't recognize anything in your photo stream, and when you view your photos through the app you see everything, whether it has notes or not. Photos with notes have a subtle page curl at the bottom right, but it would be better if the app only showed you photos you've tagged with notes rather than everything.