From The Apple Wiki

This is the XMM 6180 baseband provided with iOS 5.0 beta 6, 5.0 beta 7, 5.0 GM, 5.0.1 beta, 5.0.1 beta 2 and 5.0.1. It has both the AT+XAPP Vulnerability and the interposer exploit patched, therefore can not be unlocked. This baseband also contains the 2.13 bootloader, like iOS. This baseband can be found in the 5.0 IPSW for the iPad 2 (iPad2,2) and the iPhone 4 (iPhone3,1). It is also in the iPhone 4S IPSW due to a goof on Apple's side.