The United States Postal Service plans to issue Peanuts postage stamps sometime in 2015, according to this article from the Washington Post from a couple weeks ago.
Assuming the Postal Service doesn't change its mind, this will be the second time Peanuts characters have been featured on a United States stamp. The first was a 2001 stamp (back when postage stamps were only 34 cents) featuring Snoopy as the World War I Flying Ace.
While no details have been announced yet - and probably won't be until 2015, since stamps aren't officially revealed until much closer to their release - I'd guess that this time more of the Peanuts gang will get in on the action. Maybe if we're lucky, it will be a sheet of stamps with several different designs on it… we'll see next year!