Griffin Technologies has released a number of accessories designed to be compatible with the iPhone 3G. These accessories are now shipping and include:

WindowSeat ($29.99) - a windshield/dashboard mount which includes an audio cable for playing through the car stereo; includes an adjustable mounting bracket and works with the Griffin PowerJolt auto charger or iTrip Auto FM transmitter.
Reflect ($24.99) - a polycarbonate shell cover for the front of the iPhone device; includes a black, rubberized back case, a static-peel screen protector, and premium cleaning cloth.
iClear ($19.99, $29.99) - transparent polycarbonate case available in two models (original iClear and  iClear with belt clip and armband); includes a static-peel screen protector and premium cleaning cloth.
Streamline ($19.99) - an armband and clear, full-face screen protector with reflective trim.
Wave for iPhone 3G ($34.99) - a wave-shaped, double-helix polycarbonate case; available in complementary mix & match colors: Green & Black; Pink & Blue; Pink & White; Blue & White.