Just updated the Japanese site for Super Mario Galaxy and they have all the power ups on there with pictures of each:
Bee Suit - Mario will be able to fly like a bee for a short time. It will also let Mario land on flower petals that he would normally sink through and climb up honeycombs. It has been revealed that Bee Mario will be able to defeat enemies with his stinger. Bee Mario, however, will transform back into Mario either after taking damage, or after touching water.
Boo Suit - Mario can turn him invisible and pass through grid walls and levitate. Mario can attract other boos as well and handle them for certain purposes.
Rainbow Star - This star causes Mario to turn invincible. When Mario uses the star, he becomes a rainbow color and leaves a trail of light behind him whenever he runs.
Ice Flower - Turns Mario into Ice Mario, which lets him cross water and lava by turning it into ice.
Fire Flower - While much is implied by this flower, everything about it is not yet known. But we do know that Mario will be able to throw fireballs at items and enemies.
Spring Star - This gives Mario boosts into the air. However, these don't send Mario flying into space with power. They simple give him a small fling into the air to reach other things.
Here's the video that shows all of them off too: