Yeah, for some reason they didn't populate those headers on the ones manufactured by Adafruit, who makes the arduino.cc's Uno since the .org/.cc split. That's unrelated to the Rev3 thing, which refers to different hardware revisions.
You can easily solder a header on there. I've used a header with the pins bent in a little to get a good contact inserted in the holes without soldering for a temporary jumper in a different application but I wouldn't count on it for long term reliability, much better to solder for this application where you need the jumper left in place and getting good contact.