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 am asking this because I observed that there were some reports regarding problems with baseband 06.15.00 on iPhone 3G like no GMS connection or poor or no GPS functionality.

From my knowlege Redsn0w is the tool to do this.
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.