"Speak to" versus "speak about"

[MYR]

It seems that more and more learned people no longer "speak about" a topic or issue, but "speak to" it.
I was always under the impression that they are speaking "to" the listeners, and speaking "about" the issue.
Am I correct, or is this usage of "speak to" acceptable?

[Tdol]

I would always speak to people and about topics.