Here's a bit of a surprise. Checking on Apple's iPhone Bumpers, the iPhone 4 cases, we see that shipping is delayed for several colors. While black is listed as being available on June 24th, pink, white, orange and blue list July 2nd as their shipping dates. The green model won't ship until July 16th.
Apple's iPad case also sold extremely well at launch, mostly because there weren't a lot of options and it was so convenient to add it to your purchase. We assume the same thing is happening here. The Bumpers reminded me of iPod Socks, which were kind of silly. But hey, what do I know? Apparently they're selling very well.