I think sometimes in speech it is possible to use "about" in place of "speaking of." Example:
A: I worked all day yesterday on my English homework.
B, who told A the wrong due date: About that.../Speaking of that...
A: I hate homework.
B: Speaking of homework, have you done exercise 3 yet?
I don't like "about" in this case. I think it has to do with the fact that A's "homework" is general. He is not specifying any homework in particular. B picks up the general topic and then narrows it to a specific homework assignment.
In my example, both A and B are discussing the same specific homework assignment.