This is still very much a rumor at this point, so it's probably best to take it with a pinch of salt. According to the rumor, Apple is working on an all-silicone version of the iPhone 4 bumper, which is expected to be released after September. The rumor also claims that apple considered including the bumpers as part of the iPhone 4 package, but decided to ditch the idea at the last minute. That being said, considering that that Apple offers many color options for the bumper casings, it would be quite troublesome for Apple to include them in the package itself. Considering that the rumor claims that the new bumpers will be less expensive to produce, it's still unclear if Apple is planning to sell them at a lower cost.