Reverting back to 4.0.1 for the iPhone 4

[Karbon]

Certain firmwares update the modem firmware, so even when you revert to older device firmware the modem firmware won't downgrade.

I'm not sure if 4.0.2 does this, but it's generally why they tell people not to update until the next unlock is out etc.

na that was 4.0.1. 4.0.2 was just the PDF exploit that allowed the jailbreak through safari.