Code in header files will always be inlined?

I just had a discussion with a coworker concerning code in header files:

He says that code defined in header files will always be inlined by the compiler (like the code from the function GetNumber() in my example header). I say it will be inlined sometimes, whenever the compiler decides to do so. So which one of us has to bring a cake to work for telling filthy lies? Or maybe we are both wrong...?

MyClass.hpp

   class MyClass
    {
    public:
    MyClass();
    ~MyClass();

    int GetNumber() const 
    {
     //...; 
     return m_number;
    };

    private:
    int m_number;
    };