Update: the source has been cleaned up a bit, and the project has become inactive.
When you first enter this strange new world, everything is flat with a few raised square islands...simple enough. But as you dig deeper, be careful, for the massive spherical cave systems!
I was working on adding small spherical caves to a clean room nether generator I was working on for my own server, but I never expected large caves like these to generate.
This is my first world generator, please be easy. I'm new at this. :-}
-Large interconnecting spherical caves below ground
-Surface nether wart populator
Add this to your bukkit.yml:
worlds: [the name of your world]: generator:NetherBasic
(The plugins name is NetherBasic in plugin.yml, sorry)
Source is here: http://dl.dropbox.com/u/12637402/FlatLandsNetherSource.zip