I'm posting this in the off chance that you've already started working on upgrading Forge's sprite extension system to 1.2. I had a go at it, using a three-way compare between 1.1, 1.1 with Forge, and 1.2. I got it to compile, but I'm getting this when starting Minecraft (from MCP):
[13:53:54] Mod Initialized: mod_Millenaire 2.4
[13:53:54] Mod Loaded: mod_Millenaire 2.4
[13:53:54] Done.
[13:53:54] Loading: net.java.games.input.OSXEnvironmentPlugin
[13:53:55] ########## GL ERROR ##########
[13:53:55] @ Pre startup
[13:53:55] 1286: Invalid framebuffer operation
[13:53:55]
[13:53:55] Starting up SoundSystem...
[13:53:55] Initializing LWJGL OpenAL
[13:53:55] (The LWJGL binding of OpenAL. For more information, see http://www.lwjgl.org)
[13:53:55] OpenAL initialized.
[13:53:56]
[13:53:56] ########## GL ERROR ##########
[13:53:56] @ Post render
[13:53:56] 1286: Invalid framebuffer operation
http://dl.dropbox.com/u/14307461/millenaire/FSE1.2.zip