I heard from one of the players that while most mods are illegal the ones that inhance framerate and generally help computer run minecraft like Optifine are alowed. Just wanted to double check with before using it.
I also believe mods like Rei's Mini Map and ModMacros are allowed, but I'm not sure if they have been updated to 1.4 yet.
ModMacros looks pretty useful, but I don't want to get it until someone has confirmed that it is allowed...
Rei's Minimap works on 1.4.2 . I was told by a mod (can't remember which) that Rei's was fine (And started using it since that time).
