There's no way to add or delete pages from a PDF within the Preview application. However, if you need to make only a one page PDF document, here's how. Use the Select tool to mark the whole area of a given page, then select Edit -> Copy, then File -> New From Clipboard. The result will be a new one-page PDF that you can then save.
[robg adds: I thought that this would simply make a screenshot-like capture of the PDF, but it doesn't -- it's a real, scalable PDF.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=2006112209002019