I wish that thread had progressed to a more complete conclusion - I've been developing software with Twisted Python recently (a notoriously callback-heavy framework) and that syntax would clean up an awful lot of code in our code-base. In particular, I like that it's not a knee-jerk emulation of a particular syntax-feature of another language, but a general solution that works well for Ruby-style maps, defining property getters and setters, and setting up design-by-contract semantics.

I'd happily swap that syntax for some of the other features modern Python has grown, like the "with" statement, or decorators.