Apple routinely deletes threads from discussions.apple.com when their main purpose is to complain about Apple or Apple products. The moderators seem to act as though the forums are for people asking questions and getting answers, and not just piling on complaints. Since there's no freedom of speech on someone else's dime, that's the way it goes. There are plenty of other forums on the internet where you can complain away about Apple issues to your heart's content -- including TiPb's own iPhone forums.
(You're better off using apple.com/feedback/iphone to send in complaints anyway, there's no indication discussions.apple.com holds any weight with Apple when it comes to issue tracking.)
The internet buzz concerning the thread removals is important for one reason, however -- it shows how sensitive customers are to the antenna issue. Never mind that Consumer Reports said they couldn't recommend iPhone 4 in one sentence then named it the best smartphone on the planet in another. Never mind we still don't know how many people have the problem. Never mind that other phones have the same problem albeit to lesser degrees. Never mind that Apple's historic misrepresentation of signal strength might be exacerbating the issue, as could AT&T's network. Never mind anything else.
The antenna issue has become a matter of public perception now and that's a huge problem for Apple. The mainstream media has their teeth in it now. They smell blood. The open letter and its contents clearly weren't enough this time. As brilliant as they are in marketing, handling customer-facing problems is their kryptonite. MobileMe's initial launch might have been the goofy red kind of kryptonite. They finally acted, they finally fixed it. This might be the green kind of kryptonite. The kind that hurts.