$ gs -q -sPAPERSIZE=letter -dNOPAUSE -dBATCH -sDEVICE=pdfwrite -sOutputFile='/somedir/output.pdf' `ls -rt`
Things for dorks like me to note: 1. gs runs from your current working directory, so you have to be inside the directory you are ls-ing. 2. probably best to write the OutputFile to a different directory than the one you are in so as not to cause problems.
Why would I want to concatenate a bunch of pdfs?
So I could print them out double-sided to alleviate my guilt over printing them out in the first place.