The new Preview.app allows you to select a region of a given PDF or image for cropping or copying. If you select a segment of a PDF and copy it, you can then select File -> New From Clipboard (Command-N). This results in a full-quality PDF of the region you selected. This is very useful for citing diagrams from PDF documents without sacrificing quality. I use OmniGraffle to polish all my documents, and it allows you to include PDFs in your document at full quality. It even allows you to scale and rotate them without sacrificing print resolution.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20031103175833693