What iPhone do you have? (V1 or 3G) I'm assuming 3G, since you used ultrasn0w.
If it's a 3G try turning of 3G in (settings - general - network) if you haven't tried that already.
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White iPhone 3GS 32GB
Firmware 3.1.2 jailbroken with Pwnagetool 3.1.4 on my PC running OS X Leopard & Windows 7 Ultimate!
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