Editing PDF (or ps) files is somewhat tricky, but Leopard's version of Preview is a handy tool. It can not only extract and/or rearrange pages, but also do some more sophisticated operations, like removing unwanted margins.
Individual page cropping is very simple -- press Command-3 (Tools » Select Tool) to invoke the Select tool, then drag out the area you wish to keep, then press Command-K (Tools » Crop) and you're done. But what if you want to crop every page in the document in the same manner?
To apply the defined crop to all pages in the document, first set drag out the area you'd like to keep, as above. Then press Command-I (Tools » Inspector) after selecting the area to crop. In the Inspector panel, select the 'ruler' tab, then click the Crop All Pages button. Save your document when you're done.
Remember that it's very easy to convert any document to A PDF in OS X; just use the Preview button, or the PDF » Save as PDF button. I use this technique to edit djvu files, for example.
Mac OS X Hints
http://hints.macworld.com/article.php?story=2009020813122640