Okay, normally I'm able to solve these kind of problems on my own, but I have no idea on this one. I recently installed the recommened version of FML (4.0.0.204) and have NO other mods installed. Clean minecraft.jar, everything vanilla. I always test out each "basemod" individually as to figure out bugs between them. This is my error log: 
---- Minecraft Crash Report ---- 
// Uh... Did I do that? 
  
Time: 8/13/12 11:46 AM 
Description: Failed to start game 
  
java.lang.ArrayIndexOutOfBoundsException: 4018 
	at org.objectweb.asm.ClassReader.readUnsignedShort(Unknown Source) 
	at org.objectweb.asm.ClassReader.accept(Unknown Source) 
	at org.objectweb.asm.ClassReader.accept(Unknown Source) 
	at cpw.mods.fml.common.discovery.asm.ASMModParser.<init>(ASMModParser.java:35) 
	at cpw.mods.fml.common.discovery.DirectoryDiscoverer.exploreFileSystem(DirectoryDiscoverer.java:85) 
	at cpw.mods.fml.common.discovery.DirectoryDiscoverer.exploreFileSystem(DirectoryDiscoverer.java:74) 
	at cpw.mods.fml.common.discovery.DirectoryDiscoverer.exploreFileSystem(DirectoryDiscoverer.java:74) 
	at cpw.mods.fml.common.discovery.DirectoryDiscoverer.discover(DirectoryDiscoverer.java:38) 
	at cpw.mods.fml.common.discovery.ContainerType.findMods(ContainerType.java:30) 
	at cpw.mods.fml.common.discovery.ModCandidate.explore(ModCandidate.java:44) 
	at cpw.mods.fml.common.discovery.ModDiscoverer.identifyMods(ModDiscoverer.java:101) 
	at cpw.mods.fml.common.Loader.identifyMods(Loader.java:282) 
	at cpw.mods.fml.common.Loader.loadMods(Loader.java:402) 
	at cpw.mods.fml.client.FMLClientHandler.beginMinecraftLoading(FMLClientHandler.java:133) 
	at net.minecraft.client.Minecraft.a(Minecraft.java:402) 
	at net.minecraft.client.Minecraft.run(Minecraft.java:734) 
	at java.lang.Thread.run(Unknown Source) 
  
Relevant Details: 
- Minecraft Version: 1.3.2 
- Operating System: Windows 7 (x86) version 6.1 
- Java Version: 1.7.0_04, Oracle Corporation 
- Java VM Version: Java HotSpot Client VM (mixed mode), Oracle Corporation 
- Memory: 449839904 bytes (429 MB) / 518979584 bytes (494 MB) up to 778502144 bytes (742 MB) 
- JVM Flags: 2 total; -Xmx768M -Xms512M 
- LWJGL: 2.4.2 
- OpenGL: AMD Radeon HD 6520G GL version 4.1.11156 Compatibility Profile Context, ATI Technologies Inc. 
- Is Modded: Definitely; 'forge,fml' 
- Type: Client 
- Texture Pack: Default 
- Profiler Position: N/A (disabled) 
  
Again, it's vanilla except for FML.