Wow, this is a great program! I've been fooling around with it for a few hours and you've shaved litterally DAYS of build time off my map project. I do have a few questions and suggestions, though.
Does the current version support the painting/erasing of Flood Plains and Oasis features? I can't find that anywhere. If this is something to be added in the future, may I suggest renaming the Tree/Jungle tool to Terrain Features and adding Flood Plain and Oasis buttons there?
It's nice to be able to see the text windows at the top of the screen, but is there a way to edit that text directly in MapView? That would be a lot more convenient than switching back and forth between MapView and Notepad.
A mild annoyance is that the Save function adds redundant file extensions. It appends another ".Civ4WorldBuilderSave" to the end of the filename, even it it's already there. There are several ways to work around this, but it would be nice if that get's fixed once you get all the important stuff out of the way.
I've also seen the vanishing act mentioned by Junuxx, but only on my laptop with a GeForce Go 460 chip (and somewhat older drivers). I don't experience it on my desktop with the 6800GT (and current drivers).
Anyway, you've done great work on this program. Thanks a ton!