open -a Print\ Center /path/to/file_to_print.pdf
[rob adds: We've covered printing from the command line via lpr a number of times, but I don't think we've talked about printing via the Print Center from the command line. This works just like lpr /path/to/file_to_print.pdf, but you'll see the Print Center open, showing the spooling status of the job (assuming you're local to the Mac in question, of course!).]

