I found the easiest, earliest level to do this on was 1-10. You'll see 2 pigs, perched on top of two towers on seperate 'planets'. The blue glowing circles around each planet represent their gravitational fields. You need to get a bird stuck, where they overlap. I simply fired a bird, gently, straight ahead and it got pulled in without coming out the other side. The achievement popped once the bird had settled.
Video credit to AngryBirdsNest.
http://en.wikipedia.org/wiki/Lagrangian_point if you're interested, it's a neutral point where 2 objects gravitational fields and the centrifugal force required to orbit them, cancel themselves out. Named after some French astronomy guy.
