It'd be kinda amusing watching someone go through all the trouble to try.As Intel security architect David Grawrock mentioned during his LaGrande architecture course at IDF, you won't see too many people snooping your front-side bus with a logic analyzer.
Of course, buried later in the article they mention the ability to encrypt data going over the FSB
If a secure process has to run by itself on a CPU, I would think that it would make sense to have a system w/ multiple processors (maybe dual core?) so you can keep everything else you like to run in the background going. I don't want to sign off AIM just to submit a credit card order. Of course, they could implement some sort of protection on the resources within a CPU as well so something like Hyperthreading could keep AIM running too. Then they probably need to start protecting the stack and all too which could be a royal pain in the butt.