Do you have the option to have your poster printed on fabric? We've started doing that lately-- fold it up and put it in your suitcase, iron it out when you get to the destination. The only caveat is that some images do not appear as crisp as when printed on semi- or full- gloss. I figure that there is a 10% chance that I'll leave the poster tube on any given flight (they tend to fall into a little divot at the back of the overhead compartments), and it's worth the ~5 minutes of ironing to prevent this.

If you want to carry it on, put it in a poster tube and duct tape some string around it so that you can carry it over your shoulder. Nobody will give you a hard time if it is your 3rd piece of carry-on luggage.