Well the NE (according to Tectonicus, not game) corner I believe is the border and the location for Tribble's Claim. (Don't think he is too sure about what he needs to do regarding posting the claim on the forums, but he did clearly mark our the borders (unfortunately not clearly visible on the map, but when you are over there it is)
The Tundra to the SW of my place has been explored to the border in a few places so the western border has been found, so you can easily extrapolate the NW corner of the border from the the available data.
So the only edge I'm not sure about is the southern border, and I don't have the time to explore it.
Also be very careful exploring deserts as there seems to be a weird bug where some random Cacti are invisible, I thought it my just be a bug with my texture pack or mods, however Raennik ran into some as well when exploring the deserts to the north.
I managed to die up there which is why I didn't explore any further.