I recently needed to quickly combine a handful of PDFs into one multi-page file. I found a quick trick that will work for PDFs (and other graphic files, too). Just open Mail and compose a new message. Then drag all the files into the new blank message. Mail will render them right in the email message. Then choose Print and save to a PDF file.
The only downside is you get your return email address at the top of the first page. I suppose you could create one blank page pdf as a spacer page then print from page two onwards to get a clean multi-page PDF file.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030523181432858