It looks like the Movable Type folks have gotten themselves, and their users, into a real pickle, with somewhat ambiguous licensing terms; it isn't at all clear what's allowed, and some users have been surprised.

You'd think that using stuff licensed as free software, less featured though it may be would save you from this. Regrettably, you'd be wrong, and even use of fairly standard free software licenses won't entirely save you. The Free Software Foundation's Gnu Public License in particular has some clauses which are ambiguous enough to be in dispute, in part because it's not clear how, or whether, they apply to types of software, like blog authoring tools, which didn't exist at the time it was written. And it's not clear what will show up in GPL version 3 -- something prospective authors of GPL software might want to bear in mind before using the FSF's suggested "GPL v2, or any later version" language in licensing their software. Linus Torvalds in particular is rather pointed about licensing his work under GPL v2 only.

The difficulty here, then, isn't related so much to the freedom, or otherwise, of the software, as to whether the users can be confident they know what the deal is up front, and that the terms won't be changed later, for example, to bar competitors to some commercial venture of the author's, even if the author only deems it to be a competitor after the fact. And that's a problem that goes well beyond blogging tools.