Now I have no source to quote, but I have heard this rumour before. The idea is that in Vista as the placement to data into RAM is random, it is harder for a malicious piece of code to find the Vista back end stuff and screw it up. I'm sure (if this is true) there will be some system tracking the location of the different pieces of data (as the OS should anyway) so performance should not be affected much if at all.
If it's being tracked, then malicious code will find the tracking and be able to modify any memory address it wants to, nullifying any security bonus.
