There have now been three hints on combining PDFs (1, 2, 3). However, they have all required a using an arcane command in the terminal and installing extra Unix software.
By far the easiest way I've found to combine, split, and re-order PDFs is with a piece of software called Combine PDFs. This software will even let one set the "Title" and "Author" attributes of a PDF. Best of all, it's free, small, and very quick. Hope this helps!
[robg adds: I think we've now covered the subject of combining PDFs from just about every angle ... please post any future ideas as comments to any of the existing hints on the subject.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030903113116961