After encrypting the boot drive on a Mac system with Boot ROM version MBP91.00D3.B02, the system will no longer boot and will only display a gray screen. This problem only occurs on systems with Boot ROM version MBP91.00D3.B02 and no other Boot ROM versions have exhibited this issue. If the system is not using this Boot ROM version, the system will boot properly after PGP Whole Disk Encryption.
Apple has posted an EFI update for the Macbook Pro 9,1 and 9,2 that contains the following Boot ROM: MBP91.00D3.B08
With this Boot ROM on the Macbook Pro, the machine is able to reboot normally after encryption. This updated EFI can be found at http://support.apple.com/kb/HT1237.