Is it safe to put ipad baseband 06.15.00 on iPhone 3G in order to unlock it when using iOS 4.2.1?
I believe the iPhone 3G and the iPad 3G use very different baseband processors and putting the firmware from one on the other, if possible, would probably brick it. It's akin to trying to put the BIOS for a Toshiba computer onto a Dell.

Then again, I'm not entirely sure on this one.