Taking a full screenshot is very easy on the new iPhone software. Just press both the Sleep/Wake button and Home button at the same time. You will see a “flash” and the screenshot will be saved on your Camera Roll.
If you are writing blog posts (like me) or tutorials, you can then annotate those screenshots in Photoshop or Skitch. I especially like Skitch – I also liked Jing’s annotation tools and simplicity of use but Skitch allows annotations of photos other than screenshots. Jing does not.