Here's the net conclusion of my email exchange with Sonnet tech support:
Me: Are you actually saying that I can use either the Sonnet Thunderbolt Echo or the Apple Thunderbolt display BUT NOT BOTH SIMULTANEOUSLY?
From: Support <support@sonnettech.com> That is correct. That's why I was pointing it out to you. Didn't want you to buy everything and then discover you couldn't use them simultaneously. Unless Apple has fixed something in their TB Display, it's not going to work. That's why we have the FAQ posted.
Me: Are you actually saying that I can use either the Sonnet Thunderbolt Echo or the Apple Thunderbolt display BUT NOT BOTH SIMULTANEOUSLY?
From: Support <support@sonnettech.com> That is correct. That's why I was pointing it out to you. Didn't want you to buy everything and then discover you couldn't use them simultaneously. Unless Apple has fixed something in their TB Display, it's not going to work. That's why we have the FAQ posted.