You can use this tutorial to downgrade to iOS 4.3.3 in order to jailbreak using JailbreakMe. The Windows version of this tutorial is here.

Step One
Download and install TinyUmbrella from here if you havent already.
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Step Two
Download the firmware you want to restore to from here and save it in a Pwnage folder on your desktop. Remember you must have saved your SHSH Blobs for this firmware already.
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Step Three
Launch TinyUmbrella from your Applications folder.
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Step Four
Connect your device and make sure it lists saved SHSH Blobs for the firmware you are restoring to. If you dont see any blobs you may need click the Save SHSH button to load them from Cydia.
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Step Five
Click the Start TSS Server button.
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*Do not continue if TinyUmbrellas TSS server is not started successfully.

Step Six
Launch iTunes by clicking its icon in the dock.
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Step Seven
Place your iPhone into DFU mode by holding down the Power and Home buttons for 10 seconds then release the Power button and continue holding the Home button for another 10 seconds.

You can find help with DFU mode here.
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Step Seven
Select your iPhone from the list of devices on the left, hold down the Option key and click Restore.
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Step Eight
Navigate to the Pwnage folder on your desktop, select the ipsw file you downloaded and click the Open button.
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Step Nine
iTunes will now restore to the lower firmware version.
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Step Ten
If you happen to get an Error 1 or you get stuck in recovery mode, you will need to use FixRecovery to get your device out of recovery mode.
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Download FixRecovery from here and save it to the Pwnage folder on your desktop. Double click to extract the archive.
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Step Eleven
Place your iPhone into DFU mode by holding down the Power and Home buttons for 10 seconds then release the Power button and continue holding the Home button for another 10 seconds.

You can find help with DFU mode here.
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Step Twelve
Double click to launch the fixrecovery43 executable from the fixrecovery-osx folder we just extracted.
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Step Thirteen
FixRecovery will perform its magic after which your device will be successfully rebooted.
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IMPORTANT:
● If fixrecovery does not work the first time you may need to rerestore to your desired firmware version and try again. Remember to keep the TSS Server running during your second restore as well.

● This tutorial does work to get your from iOS 5 beta back down to iOS 4. Although some have reported less issues with a lower version of iTunes this tutorial was tested to work with iTunes 10.5b77.