"If it's the architects or BAs writing the tests, then they might actually have _more_ information about how the system should be working than the programmers,"
Oh, absolutely. I didn't mean to sound anti-non-programmer.
I tend to distrust labels like 'architect' and 'programmer'. Really there's only two kinds of people who can work on something: those who are paid to work on it, and those who have some other (usually richer and more nuanced) motivation to do so. When I said, "Organizations which separate those two functions tend to gradually suck", I was implicitly assuming both sides were paid employees.
If non-employees (I'll call them, oh I don't know, owners) contribute towards building a program the result is always superior. Regardless of how they contribute. They're just more engaged, more aware of details, faster to react to changes in requirements (which always change). Your idea sounds awesome because it helps them be more engaged.
But when it's all paid employees and you separate them into testers and devs, then a peculiar phenomenon occurs. The engineers throw half-baked crap over to testers because, well, it's not their job to test. And test engineers throw releases back at the first sign of problems because, well, it's not their job to actually do anything constructive. A lot of shuffling back and forth happens, and both velocity and safety suffer, because nobody cares about the big picture of the product anymore.
(Agh, this is not very clear. I spend a lot of time thinking about large organizations. Another analogous example involves the patent office: http://akkartik.name/blog/2010-12-19-18-19-59-soc. Perhaps that'll help triangulate on where I'm coming from.)
(BTW, I've always wondered: what's that cryptic string in your profile?)