I suppose I should start with that quintessentially bloggish idiom of a clever phrase where every word is a hyperlink detailing the community disaster they have on their hands. I might mention that every link there was taken from the trackback of the corporate weblog where they announced this "improvement"; the top title right now is "Looks Like I'll be Dumping MovableType Soon", and they get worse from there.
I feel bad for the Six Apart team. It must be very painful to be dragged over the coals like this for a change that they perceive as an improvement, and one which they are already rushing to fix. But there is an important point here, not to be lost among the recriminations and the loud declarations of switching.
In Mark Pilgrim's dramatically titled Freedom Zero posting, he explains what's going wrong. Essentially, "free enough" is not free enough. When MovableType was initially released, the licensing structure was not generally perceived as a good thing. It was a shameful and annoying legal detail that MT fans tried hard to ignore because the functionality was tempting.
What's happening here is that Six Apart has taken this small hypothetical problem and made it large, real and relevant for all their users. Although users may complain about the details of the licensing strategy, the real issue is that if it's changing now, it will change again. The message getting sent is: if Six Apart feels that you can afford more money for their tool, then they will cut you off from future upgrades if you don't agree.
This is a particularly bad idea in a market where free alternatives are both plentiful and arguably superior. Blogging tools are largely a matter of taste, since there are dozens of them with comparable feature sets.
I hope that this is a precedent, though, for more and more users to start being vocal about the fact that source secrecy, license key managers, and the like are not features, but barely-tolerated impositions. It may be that some categories of software will always be closed-source, but maybe we can work towards an understanding that that is a tradeoff.