HI yesterday I downloaded this terrrain Editor, works fantastic and do very very nice terrains...
Only a trick, when you export the mesh, go to Blender or another 3D Tool, and apply an optimizer modifer, you can drop down the polys a lot...
I do a 33.000 polys terrains and the iPad 1, runs, but a bit slowly...
http://www.bundysoft.com/L3DT/downloads/standard.php