Details On The Next Postage Increases

The cost of mailing a letter will go up on May 14, but you'll be able to lock in that price — no matter how rates rise in the future — by buying the new "forever" stamp.
[T]he 41-cent rate is for the first ounce, but each additional ounce will cost just 17 cents, down from the current 24 cents.

Full A.P. article, SF Chronicle, March 19, 2007