Mainframe operating systems, while proprietary, still have some key advantages in several areas. "The operating system provides the efficiency, isolation, the address spaces, the encryption, and supports an efficient clustering model," says Rao. The mainframe operating system is also the most trusted platform for doing key management, he says. Buffer overflows on z/OS are unheard of, says MIB Group's DiAngelo, adding that "it's a hell of a lot easier to secure one box."

But the biggest issue remains what to do with the more than 40 years of mainframe code -- much of it tightly woven into the mainframe operating system and hardware architecture -- that needs to play in a world of distributed computing and Web services. Mainframe users are sitting on more than a trillion dollars' worth of legacy mainframe code, says Rao. Bank of New York, says Mulligan, is "dealing with tens of millions of lines of code." And that amount of code couldn't be ported in his lifetime, he says.