I had a whole bunch of other mods when this popped up, but I added them in one at a time to check which one IC2 didn't party with.
Mods loaded: 5
ModLoader 1.2.4
mod_MinecraftForge 2.0.0.67
mod_ModLoaderMp 1.2.4v1
mod_Railcraft 4.2.0
mod_IC2 v1.90
Minecraft has crashed!
----------------------
Minecraft has stopped running because it encountered a problem.
--- BEGIN ERROR REPORT 40ea85da --------
Generated 3/31/12 4:40 PM
Minecraft: Minecraft 1.2.4
OS: Mac OS X (x86_64) version 10.7.3
Java: 1.6.0_29, Apple Inc.
VM: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Apple Inc.
LWJGL: 2.4.2
OpenGL: Intel HD Graphics 3000 OpenGL Engine version 2.1 APPLE-7.18.11, Intel Inc.
java.lang.NoSuchMethodError: ic2.api.CropCard.registerBaseSeed(Laan;IIIII)Z
at ic2.common.TileEntityCrop.registerCoreBaseSeeds(TileEntityCrop.java:685)
at mod_IC2.load(mod_IC2.java:575)
at ModLoader.init(ModLoader.java:891)
at ModLoader.addAllRenderers(ModLoader.java:189)
at ahu.(ahu.java:79)
at ahu.(ahu.java:9)
at net.minecraft.client.Minecraft.a(Minecraft.java:395)
at net.minecraft.client.Minecraft.run(Minecraft.java:732)
at java.lang.Thread.run(Thread.java:680)
--- END ERROR REPORT 1ede06f7 ----------
*Hopes this isn't because of a specific version*