Evernote just came out with Skitch for iPad. I got it and gave it a quick try. It's now my goto app for annotating screen shots, and whatever else. It's easy to use, and looks good. Basically it is a way to draw on various pictures (screen shots, camera, maps, web pages), then share them via various methods, including email; or you can save them to your Evernote account.
For some reason searching for 'skitch' in the iPad's App Store doesn't find it for me. I had to get the link to the store from Evernote's website. Searching works fine in iTune on the Computer. Weird.