"Since the boot ROM is independent of the operating system, reinstallation of OS X will not remove it. Nor does it depend on anything stored on the disk, so replacing the hard drive has no effect. A hardware in-system-programming device is the only way to restore the stock firmware."
"There are neither hardware nor software cryptographic checks at boot time of firmware validity, so once the malicious code has been flashed to the ROM, it controls the system from the very first instruction," Trammell Hudson said. "It could use SMM and other techniques to hide from attempts to detect it."