Editor Grid always visible

Is it possible to make the editor grid always visible?

As in through terrain and objects.

I know it is viewable in overdraw, but I would like to have it visible while painting/editing terrain.

I don't think that's possible. Everything is set to draw over the grid so it doesn't get in the way. If you need some sort of guide, create a transparent grid-like texture and place it on a plane, then tile and stretch the plane across your terrain, setting its Y-position to 0 (the same as the editor grid). It'll give you the exact same effect, only more control.