The current IOS5 Beta programme includes updates to the iPhone4 baseband. This new baseband version, 04.11.08, blocks the Gavey Ultra SIM as well as adding other enhancements to the modem.
If, like me, you have been using the IOS5 Beta for development testing you may also want to be able to restore back to a previously jailbreakable version of IOS 4, most likely 4.3.3. If you try and do this with the 04.11.08 baseband on your phone you will be in for a world of pain, especially if you are also using the iTunes 10.5 beta as trying to downgrade straight to 4.3.3, even if you have saved SHSH Blobs, will result in a DFU loop. Even if you can fix this, residual problems remain including a loss of your APN settings which need to be restored manually. Very annoying
However, it is possible to downgrade the baseband to 04.10.01 by downgrading back to 4.3.5
IMPORTANT NOTE - This method will only work whilst Apple is still signing the 4.3.5 firmware and its associated 04.10.01 baseband. As soon as IOS5 goes gold and Apple close the 4.3.5 signing window, this method will cease to work.
UPDATE - 17/10 @ 20:55 - Unfortunately I have just tested the signing window with iSHSH1T, and it appears as though it has just been closed. I am being refused blobs for 4.3.5 from Apple. Looks like anyone who has not got this to work is now out of luck... Sorry.
1. Download the 4.3.5 firmware from our sticky here - Official Apple IOS Firmware Download Links for iPhone
2. Ensure your hosts file is pointing at Apple. You can do this easily with TinyUmbrella. Simply open it, switch to the Advanced tab, UNTICK the option "Set Hosts to Cydia on Exit", and then exit TU. DO NOT USE TU FOR ANY OTHER PART OF THE PROCESS!
Alternatively you can edit your hosts file manually if you are so inclined. Ensure you remove all lines which refer to gs.apple.com.
3. Connect your iPhone to your PC and put it into DFU mode. We have a guide here if you need assistance - DFU Mode for Dummies
4. Open iTunes, which will recognise your iPhone in recovery mode, acknowledge the message about this, then press shift-restore (Windows) or option-restore (OSX), and select the 4.3.5 firmware you downloaded in step 1.
5. If you followed the instructions, the restore to 4.3.5 should now complete without any errors. When it is complete you will have a stock 4.3.5 iPhone4 again with a downgraded 04.10.01 baseband. Bonus!
Here are my before and after "About" screenshots showing that the baseband was downgraded (as well as the differences in About screens between the versions!).
Remember, the baseband cannot normally be downgraded, however because Apple are signing both the beta and current firmwares, and their associated basebands, it is possible, but only until the official release of IOS5.
Now you should be able to go on further (if required) and downgrade 4.3.5 to 4.3.3, IF you have saved SHSH Blobs for 4.3.3 available. Because your baseband has been downgraded to 04.10.01 you will not end up in a nasty recovery or DFU loop. Remember, there is no way to downgrade further unless you have saved SHSH Blobs. See our guide here for more details - SHSH Blobs - Frequently Asked Questions
Happy baseband downgrading fellow jailbreakers!