IBM Assured Execution Environment
Several years ago, while I was working at the IBM Almaden Research Center, we came up with a security mechanism called the Assured Execution Environment (AxE). We had implementations for Windows XP and Mac OS X. (Although AxE supports code signing as a feature, it’s not the same—in any case, this was long before code signing was known as a forthcoming feature in Mac OS X "Leopard".)
An evolved version of the Windows implementation is now available for download from the IBM alphaWorks web site.