Downgrade iPhone 3GS from iOS4 to 3.1.2 or Jailbreak iOS 4?

[GorillaUnit]

If I restore to a custom iPhone iOS 4 Firmware I get error 1600
If I restore to 3.1.2 or 3.1.3 it won't allow it because it says it isn't eligible for it.

What do I do?! Please help!

Thanks