While the $130 pack is still available to pre-order on XBOX 360 and Wii, the game and keyboard are available only in separate entities on the PS3. That means the set costs $10 more than on the other platforms.
Harmonix has addressed the situation by announcing that "the agreement terms for the region," the PS3 bundle will not be available in the US. "This does not affect European customers as the Rock Band 3 keyboard/software PS3 bundles will still be available in Europe," the announcement furthers.
It's bizarre, but we're going to go out on a limb and say that this is something to do with shelf-space. The PlayStation 3 sku of Rock Band 3 is likely to be the lowest selling (in America), and as such, retailers probably don't want more boxes cluttering their stores. It probably won't be an issue though, as retailers will probably do their own promotions - offering a $10 "discount" when the game and keyboard are bought separately. It would make sense anyway.