How can I sleep the display from Terminal?
In OS X 10.9, you can simply do pmset displaysleepnow. This will immediately turn off your display without changing any of your settings or putting the entire host to sleep.

Escalated privileges don't seem to be required (at least with recent versions of OS X), but if you get a message about inadequate privileges, you could do sudo pmset displaysleepnow.

Based on feedback from commenters, this is not available in OS X 10.8 or earlier.