This is funny, I actually had been working on exactly this over the last week too! (not in GS though).
I ended up creating fBM and layering that over the voronoi heightmaps (I used worley noise though instead) to get the erosion effect.
That link you posted looks way better though, will definitely attempt that next!

Link to fBM algorithm: http://code.google.com/p/fractalterraingeneration/wiki/Fractional_Brownian_Motion