CPU that will not execute data?

From: peter (nospam_at_nospam.com)
Date: 09/21/05 

Date: Wed, 21 Sep 2005 12:10:53 GMT

I remember once reading about new processor design to prevent the CPU from
executing data, thereby eliminating arbitrary code execution in
buffer-overrun attacks.

I haven't heard this idea mentioned for a while. What happens to it? Can
this idea be retrofitted to existing CPUs?