I can't comment on the "never been infected" claim, but the AS/400 does have an unusual architecture based on something called "capabilities" that provide support for objects and protection in the hardware (actually, the microcode). Then the OS is built on that. There is a detailed explanation of how this works in the AS/400 ancestor System/38 in chapter 8 in this book:

http://homes.cs.washington.edu/~levy/capabook/

PS - According to the Wikipedia article on System/38, IBM removed the capability-based addressing from the AS/400.