My favorite case for my iPhone 3G is the C.E.O. Premiere from Marware (US$35). It's designed to fit the iPhone 3G's thicker profile – although it could probably be 1mm thinner – and is made from premium textured Nappa leather. The C.E.O. Premiere has a beautiful perforated texture with contrasting white stitching and a permanently attached belt clip. It's important to note that the version sold direct from Marware features a magnetic closure, while the version sold at the Apple retail and online stores features a velcro closure. I prefer the magnetic version because it's silent when opened and closed. Velcro cases have been known to wake my daughter when I come to bed late and open the case to dock my iPhone.
The C.E.O. Premiere case has open corners that allow your iPhone to easily be pushed out of the case. The open corners allow convenient access to the headphone jack and the sleep/wake button without blocking the speaker. The C.E.O. Premiere comes with a clear protective film, microfiber cleaning cloth and a lifetime guarantee. Marware also makes a C.E.O. Premiere for the iPhone 2G.