Okay, let me fill in that report, since I have the same problem:
Launcher Version: 1.0.1.3
Operating System: Windows 7
Java Version: 1.6.0_29
Antivirus Program: Avast! 5.0.677
In addition to loosing all my mod blocks placed in the world, I also cannot access NEI or any other mod anymore, as if I was just playing vanilla.
When I try to downgrade, the alchemy mods cause strange item duplication and transformation glitches, where diamond pipes turn into full stacks of diamonds, diamond drills duplicate, and metal bars vanish and reappear randomly.
