I've the same problem.
Display Spoiler
Nov 22, 2011 5:52:09 PM ModLoader init
Fein: ModLoader Beta 1.8.1 Initializing...
Nov 22, 2011 5:52:09 PM ModLoader readFromModFolder
Feiner: Adding mods from C:\Users\Aska\AppData\Roaming\.minecraft\mods\industrialcraft-2-client_1.23.jar
Nov 22, 2011 5:52:09 PM ModLoader readFromModFolder
Feiner: Zip found.
Nov 22, 2011 5:52:09 PM ModLoaderMp Log
Fein: ModLoaderMP Beta 1.7.3 unofficial Initialized
Nov 22, 2011 5:52:09 PM ModLoader addMod
Fein: Failed to load mod from "mod_IC2.class"
Nov 22, 2011 5:52:09 PM ModLoader addMod
Feiner: THROW
java.lang.AbstractMethodError: BaseMod.getVersion()Ljava/lang/String;
at BaseMod.toString(BaseMod.java:179)
at ModLoader.addMod(ModLoader.java:279)
at ModLoader.readFromModFolder(ModLoader.java:1278)
at ModLoader.init(ModLoader.java:824)
at ModLoader.AddAllRenderers(ModLoader.java:186)
at aam.<init>(aam.java:61)
at aam.<clinit>(aam.java:10)
at net.minecraft.client.Minecraft.a(SourceFile:259)
at net.minecraft.client.Minecraft.run(SourceFile:629)
at java.lang.Thread.run(Unknown Source)
I previously had Minecraft 1.0 installed. I tried variously methods to rollback to 1.8.1 with MCDowngrader, MCNostalgica, minecraft.jar backups, and a wohle clean .minecraft folder from version 1.8.1 but it dont ever work!
I promise i didn't do anything wrong during installation, i tried several times!