I believe that this theory has already been in serious consideration since the Drake Equation has been established.

I read somewhere that for earth-like planets to really be earth-like, there has to be a massive gas-giant like Jupiter orbitinbg its star- for reasons to do with either orbits or meteors, I'm not sure which.

Even if those planets could support life, would it be intelligent in our sense of the word? How would contact be made?

I'm not so sure.

There are so many factors that go into our perfectly balanced earth; if any one of these factors were off too much we could not support life on earth (well, maybe cockroaches). Earth is a very unique planet. There probably are some other "earths", somewhere, but certainly not billions of them.

Factors that have to be within a narrow range:
Size of sun
Distance from sun
axial tilt
moon and distance and size of moon
earth rotational speed
earth size
earth elemental make-up (incl atmosphere and water)
earth magnetic core
distance from nearest planets
there may be other factors but that's all I can think of at the moment.