The white iPhone 4 has been as elusive as that famous white whale, but there are rumors that it may be available soon. But if you can't wait, or more likely, you've already given up on the white one and bought a black iPhone 4, you have some options.
I just got back from a cruise and I used a $25 Trident Cyclops case to protect my iPhone from sand and water. While Trident doesn't claim that the case is waterproof, it's dust (and probably splash) resistant. Indeed my phone survived a week in Mexico at exotic locations like Puerto Vallarta, Cabo San Lucas, and the kids pool on the Disney Wonder!
At the airport on the return trip, someone noticed my phone and asked me if it was the white iPhone 4! My white Trident case covers the iPhone completely (except for the apple logo on the back) and sure enough, it looks like it's a white iPhone. I explained and probably sold a Trident case in the process! The case comes in three parts... a front, a back, and a black silicone border similar to Apple's bumper. The Trident Cyclops is available in blue, yellow, red, orange, black and of course white. The white one, with the black accents, looks like something a Star Wars Stormtrooper might use!
The Trident case is nice but it provides so much coverage that it makes it impossible to fit in a dock connector... you will have to use a cable instead.
If you want to achieve the 'white iPhone 4' effect with a slim snap-on case, try the $30 CarbonLook from TuneWear. They even have a carbon-fiber look case, in white or black. And it snaps on or off, easily so you can change your style on a whim. It has a nice texture and a velvety backing to protect your iPhone 4's glass-back. Also, whether they planned it or not, they were smart enough to leave a wide enough opening for the volume buttons so it works with both AT&T and Verizon versions of the iPhone 4.
If you're planning on getting a white iPad 2 (as I am) and you want your iPhone to be color coordinated, these are a few options!