When to use std::forward to forward arguments?

C++0x shows an example of using std::forward:

template<class T>
void foo(T&& arg) 
{
  bar(std::forward<T>(arg));
}

When is it advantageous to use std::forward, always?

Also, it requires to use && in the parameters declaration, is it valid in all cases? I thought you had to pass temporaries to a function if the function was declared with && in it, so can foo be called with any parameter?

Lastly, if I have a function call such as this:

template<int val, typename... Params>
void doSomething(Params... args) {
  doSomethingElse<val, Params...>(args...);
}

Should I use this instead:

template<int val, typename... Params>
void doSomething(Params&&... args) {
  doSomethingElse<val, Params...>(std::forward<Params>(args)...);
}

Also, if use the parameters twice in the function, i.e. forwarding to two functions at the same time, is it wise to use std::forward? Won't std::forward convert the same thing to a temporary twice, moving the memory and make it invalid for a second use? Would the following code be ok:

template<int val, typename... Params>
void doSomething(Params&&... args) {
  doSomethingElse<val, Params...>(std::forward<Params>(args)...);
  doSomethingWeird<val, Params...>(std::forward<Params>(args)...);
}

I'm a bit confused by std::forward, and I'd gladly use some clearing up.