As of May 11, 2009 postage rates will increase for letters from 42 cents to 44 cents per stamp (good for 1 oz letter or your typical bill).
Postcard stamps will be 28 cents each.
Now is a good time to purchase the FOREVER stamps that the post office offers. The deal is that you purchase the FOREVER stamps now, at 42 cents each stamp and can use them at any time without additional postage for 1 oz letters, regardless of the rate change.
Some post offices only allow 2 packs of stamps to be purchased at a time, so keep that in mind.
Priority Mail & Express Mail rates will not be adjusted at this time, as the rates were raised in January.
In the future, additional "adjustments" as the USPS calls them will take place in May.