Alan just sent me a link to Mark Pilgrim's Freedom 0 [dive into mark] post on the whole MovableType3 thing. It's a really good read, and is pretty much in sync with my thoughts on the matter.
I've been thinking about this for a while (even before MT3). What we've got now in "blogspace" is essentially a monoculture.
Just about everyone runs MovableType. It makes a nice, big, juicy target for spamroaches/rat-bastards. Same problem in the OS world - Windows makes a nice juicy target because it's essentially a monoculture. The folks who are in the "smaller", secondary tools, (Blosxom, WordPress - for now, MacOSX, etc...) are somewhat sheltered, because they aren't the low hanging fruit. Much bigger ROI for spammers to focus on the dominant tools(s), and forget about the little guys.
Combine that with better security on the little guys, and it makes a pretty compelling offering...