This is an issue where my brain is really stuck right now. I've managed to generate a world that looks great from a distance, but as I zoom in the borders in between the tiles form straight edges etc. Could you describe more on how one can zoom into some kind noise function and genereate the noise there with a better "resolution"?
Zooming in and out just becomes a matter of showing (or generating) more detail (finer sampling) close to the camera. This is a sort of LOD problem.
With the terrain I generated I used a geometry clipmap similar to this http://research.microsoft.com/en-us/um/people/hoppe/proj/geomclipmap/