Because such design is the first thing that comes to mind when I read apple's wording of "iPhone 3G S offers highly secure hardware encryption that enables instantaneous remote wipe.".
So such design would work and is simple enough, hence "pretty sure".
I wasn't getting snippy with you, dude. I was really wondering if you knew something we didn't beyond speculation. My comrade Stephen (author of article above) suggested pretty much the same thing.
I didn't find it snippy either. Basically my position is "if it looks like a common sense thing to do that's probably what it is, unless there is a big reason to dig deeper into it".
So such design would work and is simple enough, hence "pretty sure".