How can I split a PDF file into single pages quickly (i.e. from the Terminal command line)?

This can be achieved by using pdfseparate. You can install poppler with homebrew, by brew install poppler. This will also install pdfseparate. To split the PDF document.pdf into into single pages 1.pdf, 2.pdf, etc. use:

pdfseparate document.pdf %d.pdf

Open up the pdf in preview and then on the view menu select thumbnails. Ctrl select the pages that you want now drag and drop them to the desktop.

If you're interested in doing this from the command line, you can look at Benjamin Han's splitPDF python script to do the job. For instance:

splitPDF.py in.pdf 3 5

would split the file in.pdf into 3 files, splitting at pages 3 and 5.