Dangit. There aren't any good tools out there for merging PDF documents. I mean, there are tools, but they either crash randomly or produce HUGE documents. Or cost money. Lame.
Thank goodness for Automator. I whipped out a three step workflow for selecting PDFs, merging them, and previewing the final result. It seems to work pretty alright, although you gotta name your files alphabetically to set the order they're combined. Easy!
So, if you think it might be useful, you're welcome to download PDF Combiner version 0.1.
On a slightly tangental note, I've hosted the file on Amazon's S3 service. I was looking for an easy way to upload files, and came across S3Fox ... a handy and very useful extension to FireFox for doing just that.