Lots of people know, and have tried C++. For example there are gtkmm (C++ bindings) for Hildon out there, but somewhat sub par AIUI. There have been numerous threads on maemo-developers about it, including one today.
The Maemo 2.0 (Mistral) roadmap mentions making it easier to use other languages such as C++:
Of course, it's possible to write apps for nokia with C++. For starters, read my instructions [1]. Also, my flashcard program [2] is written in C++ and runs on nokia. Also, I think minisip was another C++ project that might or might not have converted to IT2006.
What others were hinting to is that you have a steep learning curve ahead of you if you want to write GUI apps in general and ones using Gtk C++ wrapper in particular. The basic toolkit is Gtk+. The Gtkmm is a C++ wrapper around it. Plus nokia people have created an additional destkop-related hildon widget set to accomodate portable nature of 770. The C++ wrapper for that layer is naturally hildonwidgetsmm.
The good news is if you like to learn, the skillset you pick up along the way is rather useful. With just a little care, you can successfully write C++ apps for Linux desktop, nokia, and Windows.
