EDIT 2: Now I can't get to the Mojang screen, it just goes straight to a black screen.
I have just tried going on survival to test some mods I just installed, but at the moment I can't. It just goes onto the normal dirt screen, says Loading world and under that... Converting world
No loading bar no nothing. All there is there is a dirt screen and Loading world Converting world.
I get crash reports with an attempt-to-be comedian line at the top.
Here's one about a teddy bear
---- Minecraft Crash Report ----
// On the bright side, I bought you a teddy bear!
Time: 16/08/12 14:32
Description: Exception in server tick loop
- Minecraft Version: 1.3.1
- Operating System: Windows XP (x86) version 5.1
- Java Version: 1.7.0_05, Oracle Corporation
- Java VM Version: Java HotSpot™ Client VM (mixed mode), Oracle Corporation
- Memory: 463272280 bytes (441 MB) / 519110656 bytes (495 MB) up to 1037959168 bytes (989 MB)
- JVM Flags: 2 total; -Xms512m -Xmx1024m
- Type: Integrated Server
- Is Modded: Unknown (can't tell)
- Profiler Position: N/A (disabled)
- Player Count: 0 / 8; 
I don't WANT a teddy bear!
Can anyone help me? I really want to see if this'll work. The mods I have installed are Zeppelin mod and Rei's Minimap. I am trying to load a Survival map with cheats enabled.
EDIT: I have deleted the META_INF file, for the record.
I can't find a fix for this anywhere. Come on guys, I want to play Minecraft.
Are you using risu's Modloader and Forge API altogether? 'cause i had the exact same problem, stuck on the dirrt screen, getting the "disconnected from server" error, couldn't play for 20 seconds. Then i removed modloader and left Forge and my mods and everything's okay now! before doing that, check if modloader is strictly necessary for any of the mods u're using, if not, remove it without problem and use Forge API instead.
Also, a nice recommendation is to use Magic Launcher to organize your mods and install them without messing with the main failes of your minecraft. You can get it HERE.