I think posters make great gifts. In general, most people don't have enough art on their walls; and while original art can be out of many people's budgets, posters are always affordable. (I picked up this silkscreened freshwater fish poster from Smash)
Ideally, you would get a poster framed before gifting it, but if you don't, you're still going to have to protect it somehow. One option is to roll up the poster and put it in a mailing tube. The second way is to leave it flat; it ends up being a larger gift, which can be so much fun to open.
Now, obviously wrapping a piece of paper — which is what a poster is — is not easy. To make it easier, and to keep the poster safe, place it between two large pieces of cardboard. (Cardboard boxes are a great source, or buy large sheets from an art supply store.)
Using a ruler and utility knife, first cut the cardboard to size, making it two to four inches larger than the poster on all sides. Depending on the size of your poster and your cardboard, you may have to attach two pieces of cardboard together to create a larger piece.
Once you have your two pieces, sandwich your poster between them, then tape the cardboard together on all sides. You're poster is now ready for wrapping.