This is very WIP. Since i started doing this, i was mostly aiming for a server that didn't crash. It can load plugins, it can run (very simple) ones, but not much more. I'm working on fixing compatibility - I have a roadmap of plugins to get working, in ROADMAP.
Most of this mod is a fairly straight port from Craftbukkit, but I've had to modify parts (and i anticipate modifiying more).
This is ported to 1.4, and works fine on 1.4.5
After unzipping the forge source, copy bukkit_at.cfg to forge/accesstransformers (create the folder if it's not there), and copy all the files in 'lib/' (excluding bukkitAPI.jar) into the lib folder in mcp. decompile as usual. You will also need to add the files in the 'lib' folder (excluding bukkitAPI.jar) into the projects in eclipse.
If your mod does NOT use guava, you're fine, it should work.
If your mod DOES use guava, you need to add 'guava10' before all of your guava imports. The reason this is required is FML uses guava 12 which is not compatible with guava 10. So Bukkit4Vanilla ships with guava 10 in 'guava10.com.google.xxx' rather than 'com.google.xxx'.
Speak english, and please, report it in issues!
This mod is licensed under the LGPL, the same as Craftbukkit.