Does boost::bind() copy parameters by reference or by value?

By value. 1

But you can make it copy by ref instead:

void SomeFunction(const int& value)
{
    boost::bind(..., boost::ref(value)); 
    boost::bind(..., boost::cref(value)); // by const ref
}

1 http://www.boost.org/doc/libs/1_46_1/libs/bind/bind.html#Purpose

a copy of the value of i is stored into the function object. boost::ref and boost::cref can be used to make the function object store a reference to an object, rather than a copy:
int i = 5;

bind(f, ref(i), _1);

bind(f, cref(42), _1);