Thanks to salisburymistake for the idea. Thanks to God for designing trees in the first place. Please let me know if there are other types of trees you'd like or various changes or additions.
1. You must have Python (3.x) installed, go get it if you don't http://www.python.org/download/.
2. Copy the script to the folder where your maps are (usually .../.minecraft/saves).
3. Open the script with a text editor.
4. Change the settings at the top of the file to your liking (the file includes extensive instructions). Save it.
5. Double click (or otherwise run python) on the script.
6. Load up the map in Indev and and enjoy!
Added hanging roots, fixed various random issues
Fixed some more bugs. Added root buttresses, an option to turn off foliage, an option to limit the height of the trees to the top of the map, and lights (torches for now) in the trees.
fixed more bugs. Exposed a lot of functionality. Now you can edit the trunk height (for round trees), foliage density, branch density, and greatest of all trunk width!
Now with rain forests and mangroves.
Well folks, the huge trees are working now! Check out the top post for some pictures. Also, I've added an option for roots. You can choose to make roots through everything, or have them be stopped by stone, or not have them at all.
Thanks to everyone for your words of encouragement. I'm glad you all like it!
Snake: I'll try my hand at some of those trees soon.
I like this very much but can you make it so there are the old regular trees as well?
If you look at the beginning of the script you can set the tree shape to "normal" and they'll look just like the default Minecraft trees. As with the other tree types, you can run the script and have it replace all the current trees, or plant new ones in addition to what was already there. So if you already have big trees on your map, you can have the script do another pass to add regular trees in.
Oh and I really like the huge (150) tree in the OP, but I prefer island or flatland maps and I don't have enough vertical room for them. I think I'll have to carve a huge pit into one of them with NBTForge to make a huge mother tree.