NX support is important enough that it's worth risking problems with some applications, Torvalds said. "I think most people have seen the security disaster that causes most of the e-mails on the Net to be spam. So this should be trivial to explain to people when they complain about default behavior breaking their strange legacy app,"
Hope that helps..
When it comes down to what software is allowed to do with any given page or area of memory, most CPU hardware doesn't distinguish between permission to read data and permission to execute instructions. AMD64 processors, however, support an extra flag or attribute to mark individual memory areas as nonexecutable -- the NX bit in the page table entry (PTE) -- when running in 64-bit or in 32-bit Physical Address Extension (PAE) mode.
The latter enables processors to address more than 4GB of memory, the normal ceiling for a 32-bit CPU, and its extra level of paging makes NX possible. Microsoft says some 32-bit applications and drivers may stumble when running in PAE mode, but Service Pack 2 reduces the risk of incompatibility by making hardware-abstraction-layer changes that mimic "classic" 32-bit direct-memory-access (DMA) behavior.