Update: Guido appears to share my opinion on this.
I've heard murmurs from a bunch of projects over time concerning their transition plans to Python 3.0. These transition plans are naturally vague at this stage. There's one theme I've heard a few times in these murmurs that worries me enough to speak out on it.
People sometimes reason that since Python 3 is going to break people's code anyway, now is a good time to make backwards compatible changes in their libraries too. The reasoning is understandable: people are facing a big change anyway with Python 3, so why break backwards compatibility for them once then, and then another time for your library? Better do it at the same time and make people suffer only once.
There are other reasons that drive projects to break backwards compatibility at this point. As far as I understand, the Python C API will change with Python 3, so that means projects that bind to C libraries need to be adjusted anyway. This might tempt people to make larger scale changes.
I just heard that the current WSGI spec isn't entirely compatible with Python 3. If that is so, and a new version of the spec is needed, then that might give some frameworks that use WSGI an opportunity to clean up some things and break backwards compatibility.
While very understandable, the negative consequence of all this is that the Python 2 to 3 conversion script strategy stops working for code that depends on these libraries and frameworks. This official transition strategy aims to make the transition between Python 2 and Python 3 easier by offering a script that convert your Python 2 code to Python 3 code automatically.
If lots of important libraries and frameworks however also break backwards compatibility in their Python 3 versions, this means that any converted code that depends on them has no chance of working. Instead the developer would need to track all the API changes and make adjustments there too before the code would work again: a much harder task.
So my advice to library and framework maintainers would be: please do not use this opportunity to break backwards compatibility in your library or framework too. I'd like to ask you to resist this temptation. The language changes already make this a major transition for everybody, and you'd make it that much harder by taking away people's ability to automatically convert their Python codebases.