I saw GTK+/Clutter are supported for Moblin compatibility.
I cannot find mention for Hildon support in MeeGo.
Will tons of Maemo5 applications work on MeeGo?

If not oficially supported, I don't know of any issue for hildon not compiling for MeeGo. Even if I thought hildon requiered a highly patched gtk+, also libhildon is already in Ubuntu repositories.

thanks, I want to start developing something for my N900, but I do not know C++ so I cannot use QT. I know well C so I chosed GTK

If you're starting something new, you're much better off using Qt even if it means learning C++. It's not really that difficult, and Qt simplifies memory management. I recommend starting with this book.

I took 1-3y to learn C, and about 2y to learn GTK in spare time. I estimate 1y to learn the differencies of C++ and 1y to learn QT. Seems to me is not reasonable to write a simple application only for hobby. May I can change idea if I find some rapid guide to call C++ method from C, and continue use C for my code.

C++vas designed to be C compatible. Nearly every C program is a C++ program. So you can write a C programm with invocation of C++ methods and compile it with g++ instead of gcc.
But usually it is threated as bad stile programming and doesn't look well. You soon would fin out that you use too much of C++ features and your code is a strange combination of paradigms.

if it is only a problem of stile and look I don't mind, I want to use C for my CLI/algorith code, as this is the standard, fast and very portable. The problem is that I do not know how to call a C++ method from C as now, and this is required for QT. Nokia do not officially support C language bindings, neither show some examples.
In my case (and I think in many hobbist), comparing hildon/GTK/C programming (0y training) with UX/QT/C++ (1y+ training), lead to dubt in choice.

Compile your C program with a C++ compiler, that's all you need for bindings, basically. Or create the UI with Qt and C++ from the beginning, then use C for algorithms and lower level stuff as you like.

True that you'll have to spend some time getting used to object-oriented programming with the GUI part at least, but once that's out of the way learning the basics of Qt should be a matter of days.

how to call a C++ method from C?

Directly, no wrapper needed. You C code will actually compile to C++ code directly.

Since C is a subset of C++, you can design your UI with C++, maybe even with QTCreator or other tool, and then just add your algorithm source and header files and invoke then directly from C++.

If you have in your C source/headers:
void do_something(int myint, char *cool_array, float awesomeRate);

You just call them in C++ with:
do_something(i, charArray, 3.1);

C++ is C with powers, believe it!