According to the Dumbledore's Army web site from Warner Bros., Harry Potter and the Order of the Phoenix has a U.S. rating of PG-13. The rating means that parents are strongly cautioned and that some material may be inappropriate for children under 13.
You may remember that Goblet of Fire was also rated PG-13 in the States. (It should be noted that neither the MPAA or BBFC have posted an official rating online. We have asked for confirmation from Warner Bros. on this and will update as soon as we can.)
Thanks to UHP for the update.
UPDATE: Commercials airing the trailer in the U.S. starting on Friday, May 11th feature a PG-13 rating.