One point I've not seen raised yet, which I think is the most important:
Most of the libraries I use on a daily basis are C libraries with bindings for Python, Ruby, Perl, Java, etc. From what I've seen, it's a lot easier to wrap C libraries with 19 different language bindings than it is to wrap C++ libraries.
For example, I learned Cairo once, and have since used it in 3 or 4 different languages. Big win! I'd rather write a program that can be used again in the future, and writing one that can easily be adopted to other programming languages is an extreme case of this.
I know it's possible to bind C++ libraries, but AFAICT it's not the same. I've used Qt (v3 and v4) in other languages and it's not anywhere near as nice to use: they feel like writing C++ in some other language, not like native libraries. (You have to pass C++ method sigs as strings!)
C++ is probably a better language if you're writing a function to be used once, or if you think all the world is C++. C seems like an easier language if you're designing for language-portability from the start.
